SVG2482AC with Xfinity Voice service - can make calls but cannot receive calls

I’ve been dealing with Comcast for a month to get voice service on an existing cable line. They were finally able to provision the service and send a signal to SVG2482AC so that it appears on the network. There is a dial tone and the modem will make calls. However, if one calls the number associated with the modem, the phone never rings. For the caller, it’s as though the line drops before any connection is made. No fewer than five technical people at Comcast have looked at this. They’ve re-provisioned and re-booted the modem multiple times to no avail. They have a service technician coming onsite tomorrow, but it’s not clear what they’ll be able to do. Any suggestions? I’m going to make sure the modem has the latest firmware before they arrive, but somehow I can’t believe that will help. I have to believe it’s something on Comcast’s end.