Below possible options for troubleshooting inbound telephony not working
Inbound number (DID) not bound to correct Distribution Group Check your Distribution Group IP address settings in Voice Management App.
Firewall not configured for CM inbound trunk IP addresses Inbound traffic is routed from our dedicated inbound platform. Please allow incoming traffic from 18.104.22.168 and .11
No correct response to SIP OPTIONS keep alive messages If a 404 error No Route To Destination|No More Destinationserror response is received. Our platform is sending so called Keep Alive messages via SIP OPTIONS message which need to be acknowledged by a 200 OK from the other side. This is in the principle by which our gateways check if the inbound trunk is still available.