Sonic fixed this back in August after I posted the problem on July 13 of this year under the heading: “Accession Communicator Service Issue on iPhone”. Accession was replaced with Sonic Mobile Communicator.
Now the same service issue message is appearing in the Sonic Mobile Communicator app: "Service Issue! You are not receiving incoming calls. This is a temporary issue, however if the problem persists please contact your service provider."
I have disabled and enabled Sonic Mobile Communicator within sonic.net voice settings as well as logged off and back on in Sonic Mobile Communicator with no resolution. My IOS version on my Apple iPhone is 11.4.1 (as before).