If you see a 403 error in response to difficulties placing calls see if you are also noting a Q850 response from Microsoft.
Q850 responses provide reason codes which may also carry verbose detail on the cause for the rejection.
Some diagnostics may be necessary to establish the reason why Microsoft are rejecting a call and where we can we will offer suggestions as to how these can be resolved.
Reason header: Q.850 ;cause=63 ;text="9810e5a9-667c-4513-8d3a-5d83503e1c71;Current call site state prevents fork to this entity."
Possible solution: AllowCallingPreventTollByPass Setting