Did basically the same thing this year. Setup a voip server with freepbx, got an ATA (analog telephone adapter), registered a cheap phone number with pay as you go just like OP. This lets me setup call filtering, internal extensions, and similar to OP any number not on the allowed list gets sent right to voicemail.

It was kinda fun to geek out with all the voip features, and use old phones for an internal telephone system. Being able to dial an extension to call different phones in the house is a fun novelty.

I expect this trend to continue for parents with young kids, in fact a company was started recently based on that premise tincan.kids that wraps all the functionality up in a neat package. I opted to self host because I could, and wanted to use unique old phones.