Hello @Roger Fawbert
Are you getting any specific errors when trying to add your existing domain? How did you add the domain?
If you have already added and verified a custom domain in your Azure tenant, you should be able to use it for Direct Routing in Azure Communication Services.
To add the custom domain to your Direct Routing configuration, you can follow the steps provided in the following document: Use direct routing to connect to existing telephony service
If you are still unable to add the custom domain to your Direct Routing configuration, you may need to check if the custom domain is associated with the correct Azure subscription and resource group. You can also try to remove and re-add the custom domain to see if that resolves the issue.
-Grace