Microsoft have announced, via the Microsoft 365 admin center Message ID MC307310 that they are removing the support of the sip-all.pstnhub.microsoft.com domain for Teams Direct Routing in March 2022.
Why is this important? Guidance from various SBC vendors and community leaders in blog posts has always been to use this sip-all domain when configuring the SIP trunk to Microsoft Teams. Without adding the specified subnets (188.8.131.52/14 & 184.108.40.206/14) calls are likely to fail between Microsoft Teams and the SBC.
Adding the subnets on Ribbon SBC 1000/2000/SWeLite is fairly straightforward. Within the signalling group just add the IP subnets using the green cross icon into the SIP Federated IP rows section. In my example I’ve added the subnets into my previous list of all the pstnhub.microsoft.com addresses. Other SBC venders will have a similar method but you would need to refer to their documentation.
Finally, it is important to also check the firewall rules (providing there is one) between the SBCs public IP address and Teams to ensure firewall rules do not reference the sip-all.pstnhub.microsoft.com address.