Making the most of a dumb fax switcher box in the old days

I have voice over IP via fiber from Sonic. The house's old interior phone wiring is no longer connected to the telco in any way. So Sonic's VOIP box is plugged into the wall jack for the old phone wiring, from which it can reach some old phones around the house.

The main problem with this is that the Sonic-branded box is too dumb to manage power failures properly. The fiber modem/router comes up fine by itself, but, on every power outage, the Sonic VOIP box has to be unplugged and reset before voice phone service comes back up. Incoming calls are silently lost. The problem seems to be that the VOIP box comes back up before the Internet link is fully operational, confusing the VOIP box.