In my previous post, i have described what the root cause to make external call transfers and forward fail, in Direct Routing with Teams. In that article, i have explained how to fix call forwarding by setting up manipulation rules in AudioCodes Mediant SBC.

In this post, i’m going to explain how to fix call transfer function. As the same way i fixed the call forwarding, i will use message manipulation here as well. But, i will be using a different SIP header, instead of  Call History.

If you are familiar with Skype for Business or Lync, you would know that they both use REFER to handle call transfers. Teams is exactly the same. It use REFER for call transfers. But unfortunately, most of the SIP Trunk providers does not support this function. So, we either disable REFER in on-premises Skype for Business\Lync servers, or set the SBC to take care of it locally.

But, in Teams environment, you can’t disable REFER. The Trunk configuration does not have any such parameter to alter. So instead, we set the SBC to handle REFER locally. I don’t cover that configuration here. I have already mentioned that in my previous article that explains how to configure Direct Routing with Teams, using AudioCodes SBC.

So to fix Transfer, i use REFER related headers in SBC. Idea is to manipulate the FROM User header, replace with SIP Trunk Pilot DID, if the session referred-by header contains sip.pstnhub.microaoft.com. This way, the rule will only allow to externally transferred calls.

After configuring this, the Manipulation Set ID must be assigned to the Outbound Message Manipulation setting of the IP group of the SIP Trunk.

Once this is done, the call transfer to PSTN destinations should work. In my test call below, i’m making a call from +61451xxxxxx, to my DID +61249xxxxxx and transfer to +61131313.

As expected, the call get accepted by the SIP Trunk and connected with +61131313.The Source DID of the call was manipulated to the Pilot number of the Trunk.

That fixed the external transfer issue for me.

If you still having issues with PSTN Transfer, check the SYSLOG of a failed calls to identify what could be the root cause. Post in comments if you still have issues with PSTN call transfer setup in Teams (with Direct Routing).

When Direct Routing with Microsoft Teams has been deployed, you will notice that forwarding and transferring calls to PSTN numbers might not work. But, if the PSTN connection is an ISDN trunk or anything TDM, this will work from the get-go.

The reason behind this is, both Skype for Business and Teams sends original caller ID, when a call get forwarded or transferred to PSTN. The original caller ID will be the CLID of the PSTN inbound call. If the PSTN connection is a SIP Trunk, then the trunk won’t allow outbound calls from a source number that does not belongs to it’s number range. It will reject the call.

Below is a SYSLOG capture of a call that getting forwarded to PSTN. Notice the FROM field. That is a Mobile number. HISTORY INFO carries the DID that forwarded the call.

If the PSTN connection is a TDM trunk, then it will mask the caller ID to the pilot number of the trunk and process the call. In this article, i’m going to explain how to fix PSTN call forwarding to work with SIP Trunks.

In my setup, i have Direct Routing configured in AudioCodes Mediant 800 SBC. I have a SIP Trunk from a local telecommunications provider for PSTN connectivity. So, let’s start.

1st thing you need to do is, enable “ForwardCallHistory” in OnlinePSTN Gateway. Log in to Skype for Business Online Powershell and run the command. By default, it turned off;

Set-CsOnlinePSTNGateway -Identity <FQDN of the GW> -ForwardCallHistory $True

Log in to AudioCodes SBC and create a Message Manipulation rule to manipulate the FROM User URI to replace it with the Pilot number of the SIP trunk. Unfortunately, we can’t get the actual DID number that sets the call forward as that information is not there in INVITE header. The Manipulation Set ID mentioned below can be any value. I had to use “5” as i’m already using 1-4 for different rules. Action Value should be the Pilot number of the Trunk. 

After that, set the Manipulation set ID in Teams IP group, “Inbound Message Manipulation” setting.

This should fix the number inbound number presentation in to SIP Trunk. If you look at a SYSLOG capture,it will show how the numbers are getting presented to SIP trunk. Notice that the FROM number have changed from +614xxxxxxxxx mobile number to 089xxxxxxx value, that was set in Message Manipulation. This number was manipulated to remove “+61” and add “0” by an “Outbound Message Manipulation” rule. 

By now, call forwarding to PSTN should work. If it still having issues, check the SYSLOG of a failed calls to identify what could be the root cause. Post in comments if you still have issues with PSTN call forwarding setup in Teams (with Direct Routing).

In this article, i’m going to cover the configuration that involves in configuring the AudioCodes Mediant VE (Virtual Edition) with Microsoft Teams using Direct Routing. Let’s start with the required ingredient list;

1. Certified SBC for Direct Routing. In my case, it’s AudioCodes Mediant VE, version 7.20A.156.028 (Newer Firmware versions will require dedicated Teams License in SBC)
2. AudioCodes SBC and Teams licenses
3. Public Certificate with a SAN record with the host name of the SBC (.pfx file).
4. Public IP address
5. Firewall Rules
6. Public DNS Record

SBC Deployment Topology

Just to explain a bit about the supporting infrastructure for the SBC, specifically Network layer, the SBC is configured as a multi homed devise where one interface is in DMZ and the other interface is in LAN network. The reason for this setup is to secure the corporate network from potential threats from in the internet. As one of the Gateway interfaces will be opened to the internet, it’s recommended to follow this process.

SBC it self is a secured devise. So nothing other than allowed SIP traffic can be passed on to the corporate network.

Public DNS and Firewall rules

A set of Firewall rules need to be defined, so that Teams SIP Proxy can communicate with the SBC. At Teams end, there are 3 Sip Proxies. Those are, sip.pstnhub.microsoft.com (Global FQDN), sip2.pstnhub.microsoft.com (Fail over FQDN), sip3.pstnhub.microsoft.com (Fail over FQDN). These DNS records resolve to below IP addresses;

• 52.114.148.0
• 52.114.132.46
• 52.114.75.24
• 52.114.76.76
• 52.114.7.24
• 52.114.14.70

The required firewall rules for SIP Signalling are below;

In this setup, we’ll be using the SBC Source port as port 5067.  Firewall rules that are required for Media are below;

The Media port range that we’ll be using in the SBC will be the range 6000 – 6499. The IP rage within Teams that allocated for Media is;

• 52.112.0.0 /14 (IP addresses from 52.112.0.1 to 52.115.255.254).

It also requires a public DNS record configured for the IP public address that is used for DMZ interface in SBC. The public IP address can be NAT to an internal DMZ IP or, the Public IP address can be directly configured on the SBC’s DMZ interface and configure the Firewall in DMZ to control traffic.

In this setup, as i have my SBC in Azure, in’m using a NATed public IP on my DMZ interface of the SBC. And i have a DNS record configured to resolve the host name sbc.uctechie.com, against that IP address.

SBC Network Configuration

AudioCodes Mediant VE is now available as an App to be deployed in Azure. If you log in to Azure portal, you can find the app and deploy it the way you want. In this setup, I’m using AudioCodes Mediant VE app in Azure. I’m not going to cover the steps for SBC provisioning in Azure in this article.

Let’s start with configuring the network component. The LAN adapter will be already configured and accessible by now. Configure the WAN interface with a DMZ IP address as shown below;

The LAN interface must be configured to use OAMP+Media+Control applications, while the WAN interface should only do Media+Control. Both interfaces should be in dedicated VLANs. The network connectivity should looks like below;

Next, move to configure DNS. I need to configure SRV record for Teams proxy as it uses multiple host names and IP addresses. Only way to handle this within the SBC, is to configure a SRV record and configure each proxy address within the SRV it self.  Navigate to Setup> IP Network> DNS > Internal SRV, and create a SRV record for teams.local as shown below;

Configure Certificates

As Microsoft Teams will only use TLS and it’s connected over the Internet, a public certificate must be used in the SBC to establish TLS sessions. My public certificate contains a SAN record for the SBC;

I will be using the above mentioned .pfx file of the public certificate here. Navigate to Setup> IP Network> Security> TLS Context, edit the default context entry and configure as mentioned below;

Once the configuration have been saved, navigate to “Change Certificate” in the same window to upload the .pfx file.

Before uploading the certificate, check the “Private Key Size” is configured as 2048 and not 1024. If it’s set to 1024, then change that to 2048 from the drop-down and click on “Generate Private-Key”. This process might take couple of seconds to complete. It’ll show as “New Private Key Configured” on the same window, upon successful configuration.

Now, we can upload the certificate. Notice the “Upload Certificate Files from Your Computer” section. Specify the Private Key pass-Phrase in the specified box. Click on “Browse” and load the .pfx file.

Once the certificate have been successfully uploaded, navigate back to “TLS Context” and click on “Certificate Information”. Confirm that the recently uploaded public certificate shows up with the “Status : OK”.

Go back and click on “Trusted Root Certificates”  and confirm that the Root CA and subordinate CAs for public certificate appears there.If not, just import the files in.

Configure Core Components

First thing to do is, to configure the Time and Date in the SBC. Navigate to Setup> Administration>TIME & DATE, configure the Primary NTP server and UTC Offset values. Usually, the NTP server is the Domain Controller. 

Next is to configure the SRD. There should be a default SRD configured. I’m going to use that one. Navigate to Setup> Signalling & Media > Core Entities> SRDs, configure the default SRD with highlighted values as shown below;

After the SRD, let’s configure a SIP Interface to be used with Teams. Navigate to Setup> Signalling & Media> SIP Interfaces, configure an Interface for Teams as mentioned below;

Moving on to configure the Media Realm, navigate to Setup> Signalling & Media >Core Entities> Media Realms. Change the “Default Realm” and create a new Realm as “Teams” as shown below;

Now to configure Proxy Sets, navigate to Setup> Signaling and Media> Proxy Sets, configure a Proxy set to be used with Teams as shown below;

Another bit to do here. Click on “Proxy Addresses 0 items” link in the bottom of the same window. Configure a Proxy address to connect to Teams as sown below;

Now, i need to create an IP profile to be associated with the Teams IP Group. Navigate to Setup> Signaling & Media> Coders & Profiles> IP Profiles, Create a new profile with the configuration as shown below;

Moving on to configure the Audio Coders. The required coders for the integration with Teams are

• SILK Wide Band
• SILK Narrow Band
• G.711 A
• G.711 U

By default, G.711 A and G.711 U is included within the SBC. In order to enable SILK codecs, the Transcoding license is required.

Note : SILK Codecs are recommended as the preferred codecs to be used for the integration. Missing those, G.711 A law will still work. But will not be as optimized as  using SILK Codecs. To configure Coders, navigate to Setup> Signaling and Media> Coders & Profiles> Coders Groups. Edit the existing Group _0 and add Codecs in above mentioned order

(my Lab setup does not have Transcoding licenses and i’ll be using only G.711 A and U laws. In production environment, make sure to procure the required license).

Next step is to configure an IP Group to be used for Teams. Navigate to Setup> Signaling & Media> Core Entities> IP Groups, configure the IP Group as shown below;

Media and SBC

Now we are done with the core configuration. Moving in to configure Media components. Starting by enabling Media Security and SRTP. Navigate to Setup> Signalling & Media>  Media> Media Security. Configure the parameters as shown below;

Also,

• in Setup> Signalling & Media>  Media> RTP/RTCP Settings, Configure “Comfort Noise Generation Negotiation” to be Enabled.
• in Setup> Signalling & Media>  Media> Voice Settings, Configure “Silence Suppression” to be Enabled.

We need a Message Condition Rule, that need to be assign with the Classification. TO configure the Message Condition, navigate to Setup> Signaling & Media>  SBC> Message Manipulation> Message Conditions, configure a condition as shown below;

Next to do is the SBC component configuration. Navigate to Setup> Signaling & Media>  SBC> Classification, configure inbound connections from Teams to be classified as allowed and trusted source. Configure the Classification as shown below;

Next is to configure a Route to allow OPTIONS requests to flow between Teams Proxy and SBC. Navigate to Setup> Signaling & Media> SBC> IP-to-IP Routing and configure a route as shown below;

Now, we need to do a SIP Header manipulation. The reason behind this is, the standard OPTIONS request, the FROM header consist of the IP address of the SBC it self. Since Teams only allow TLS connections, which is based on the device host name, the IP address need to be replaced with the host name. To do this, configure a Header Manipulation rule as shown below;

Now, we need to enable this Manipulation set and assign to the Teams IP Group. To enable the Manipulation set, browse to http://<SBC IP Address/AdminPage and configure the value as mentioned below;

Go back to the IP Group configuration and confirm that the “Outbound Message Manipulation Set” is configured with the proper value;

Finally, we are done with the configuration. If all works well, you should see a green check mark on “Teams” IP Group in Topology View;

Also, if you go to Monitor> Monitor> VOIP Status> Proxy Sets Status, all Proxies should come up as “Online” and the active one would be marked as (*);

Now, i’m good to make a test call from my Teams client. I’ll be calling my test mobile, which i have a route configured (in my previous blog post) to allow Mobile calls to this SBC;

And we have an inbound call from Teams, coming in to the SBC. Notice the IP address of the inbound SIP message and the User Agent header in the message. The call comes from Teams proxy server. We can confirm that we have a functional integration between Microsoft Teams and Mediant VE SBC. Please post any comments that you have in comment section below. Thanks.