Nice, and the Grandstream adapter is good to know about. There is a gadget now called cell2jack that lets you use your old land phone as a mobile handset but going to VoIP is also interesting.

Quite a while ago, Sparkfin sold a rotary phone that had been modified to have a mobile phone board inside, but it is now long obsolete:

https://www.sparkfun.com/products/retired/287

I still have an old rotary phone around, with the carbon granule mic those things had. It sounds awful. Idk whether that's due to its age, or because those mics just sounded bad compared with modern condenser mics.

I have been thinking of getting a cell2jack to have a pseudo-landline at mom's, since there are people there who get confused by smart phones.

This board also looks promising:

https://www.keyestudio.com/products/keyestudio-raspberry-pi-...

Unfortunately GSM modules in that style no longer work because the 2g and 3g networks are now shut down around here.