I have two small offices and I would like to have telephone 'extensions' both ways. Simply, POTS phone at office 1 to POTS phone at office 2. The 'PBX' at each location have POTS extension ports. I have unused public IP addresses at each location. What 'black' box will let me plug in a POTS phone into it and the other side is a network (TCP/IP) connection to my internet router. The 'black' box will only be used to 'dial' one IP address ( fixed programmed IP number ? ) at the other office. Same setup at the other office coming back. And a simple setup, just configure a single IP number/address to call automatically ( or press a dial number button) when the phone is off-hook. Same at the other office in the other direction.
POTSa > BOX > ROUTER > INTERNET > ROUTER > BOX > POTSb - and - POTSa < BOX < ROUTER < INTERNET < ROUTER < BOX <POTSb