Assuming you have adequate control over the local network to deal with jitter one could just use SIP ATA’s with everything set to use G.711 (uncompressed PCM au

Assuming you have adequate control over the local network to deal with jitter one could just use SIP ATA’s with everything set to use G.711 (uncompressed PCM audio) and make modems quite happy to believe they are on a traditional POTS line. You would need to know how to setup Asterisk or something similar to do this, but it’s not a Herculean effort either. Alternatively, POTS PBX devices exist as well and would work fine - though they’re harder to come by these days.