My guess is going to be your fiber service came with a new router. In my case my Voipo service connects to Voipo owned SIP servers. But my calls are handed off directly to the carrier. Thus the voice traffic can look like unsolicited traffic to your router firewall.. Any good firewall is going to block this. A few simple steps and my voip service is perfect here and at my customer locations. I like their method of handing off the traffic because it takes several hops out of the equation and thus less latency on a call.

You may be experiencing something similar..

What modem and router did your last service have?

What modem and router is your new service?