So you want to set up external calling in Teams. There is a lot of information out there about direct routing in Teams. Microsoft actually does a pretty good job documenting it. But one thing that gets lost in the documentation is creating an account with the new sub domain. Below are the steps to set up an SBC in your Teams environment.
Your telco has set up their Microsoft supported Direct Routing SBC. They will provide you a FQDN and port they have set up as a tenant on the SBC.
Go to your 365 Admin Center. You need to add that FQDN as a sub domain for your organization. Once you do that, you will get information similar to the below, which you will need to share with your telco.
3. Once the domain is verified by both parties. You can move forward.
4. This next step is one that gets missed. You need to create a user, using the new domain. If you don't do this, you will get an error when adding an SBC. In 365 Admin Center, add a user with the domain. You will want to think about the naming convention now, because most likely, your SBC list will grow.
5. In the Teams Admin Center. Go to Voice-> Direct Routing. Click Add and change the necessary settings.
6. Click Save
7. In the same page in Teams, click on Voice routes.
8. You'll need to set up routes. I set up 3, LocalCalling, LongDistance, International. Once they are set up. Click on the route to edit. Chose Add/remove SBCs. You will now see the SBCs you have enrolled. You need to click the boxes and press Apply.