If you have previously used Teams Direct Routing then you may have used Powershell commands such as New-CsOnlinePstnGateway to tell Microsoft about your SBC. This will have caused it to display in the Teams Direct Routing Dashboard.
Microsoft have a different model for multi-tenant carriers where the PSTN Gateways are created in the carrier tenant and do not appear in your Office 365 tenant. By adding the domains to your Office 365 tenant it allows your Voice Routes to refer to the PSTN Gateways in the carrier tenant. This means they will not appear in your Teams Direct Routing Dashboard.
There are several reasons that this multi-tenant carrier approach is better:
Microsoft only need one set of SIP OPTIONS pings per SBC rather than the hundreds that would be sent if every customer configured their own PSTN Gateways.
There is no need for you to publish DNS records for the SBC.
This approach uses wildcard SSL certificates, so there is no delay as there is no need to acquire a specific certificate for you.
The carrier can fine-tune the PSTN Gateway settings without needing you to do anything.