I had the same problem as in: Fairphone 3 to no longer work on UK Smarty Network - #14 by gapintheclouds - that the 3G switch off meant that I could not longer receive or send calls or SMS away from WiFi.
On good advice in that forum, I picked up a 3 SIM - believing this to be a major network that FP would have negotiated VoLTE access with. It does allow me to make and receive calls (and I presume SMS) if I have it set up for 4G calling (not otherwise).
However, all received calls have “Unknown Caller ID”, which actually makes it impossible for me to use (I have to have my phone on permanent DND with exceptions for vital family numbers - who now cannot get hold of me).
Is the Unknown Caller ID a separate problem (it pre-existed the 3G switch off, and it comes up in other threads, including one I reference in the above thread, so I assume so)? Is it simply that the FP3 is obsolete? 