I’d really like to see a shield for a common wireline (not wireless) phone interface. Specifically, I’d like to be able to recognize sensor input and trigger a wireline phone call to someone who cares. Conversely, the device could recognize incoming calls, decode DTMF and make that available to the controller. That would further allow control of the device from outside. But the real key is the wireline interface (DTMF decoding can be easily done with other chips). BTW - the entire device (Arduino, + shield) could be powered from the phone line.
What you’re looking for is a DAA - data access arrangement. XECOM used to sell a few inexpensive ones with FCC certification. I think they were bought out by Cermetek.
You can build your own DAA from discrete parts but I don’t recommend it: getting the audio balanced can be tricky. A simple way might be to buy a cheap eBay external modem and couple audio to it if you need to send/receive audio signals in addition to DTMF.
MultiTech has some reasonably priced OEM POTS modules that are fully certified and would provide a good portion of your solution. You’d have to add a DTMF decoder chip and power regulation circuit, but otherwise the module’s AT command interface would be easy to integrate with any microcontroller.
For a previous project I looked at the MT2492SMI-L-34.
Keep us posted on your progress!