Provided you are not behind CGNAT then dynamic DNS is your way forwards here. My parents have a RPi for a phone exchange (RasPBX) and a pair of Yealink DECT ha

Provided you are not behind CGNAT then dynamic DNS is your way forwards here. My parents have a RPi for a phone exchange (RasPBX) and a pair of Yealink DECT handsets and BT Home for an ISP. A static IP is impossible. I tie the Pi to my home router via OpenVPN and use https://freedns.afraid.org/ https://freedns.afraid.org/ for dynamic DNS to send calls to. It has worked fine for four years now. The Pi runs a one liner from a cronjob every 15 mins or so. The TTL for the DNS entry is very short and the DHCP lease is something like 24 hours plus also I think it tends to renew the last one used reasonably reliably. I don't bother monitoring that. I could write the cronjob result to a local log I suppose. More things can go wrong than the tiny window that might exist when the DNS might be out of whack. Don't let the lack of a static IP address get in the way.