You'll need some analog circuitry. Not much, but phone service has some kinks electrically speaking. A phone on-the-hook appears like an open circuit to direc

You'll need some analog circuitry. Not much, but phone service has some kinks electrically speaking. A phone on-the-hook appears like an open circuit to direct current. Off-the-hook the circuit is closed, and current starts flowing. (Pulse dialing and "flash" signals open/close the circuit, telegraph style.) And ringing on POTS is done by applying about 20 Hz AC to the line at around 100 volts -- enough to make an electric motor ring a bell, or unpleasantly surprise someone playing with the "low voltage" phone line. So you need to block the AC ringing (and other spikes), block the DC bias voltage (while presenting a phone-like impedance to the line), and then join both the DAC and ADC to the same two-wire line with matched impedance. It could be anywhere from a couple transformers and a few resistors and caps, to a very complicated arrangement with amplifiers, active noise cancelling and gain control circuits, etc. (There used to be dedicated ICs for those things.) POTS, by its analog nature, is extremely flexible in practice. A couple 9V batteries are often enough to make two old phones work as intercoms. But the faster you want to go the more important it will be to match the expectations of a POTS line exactly.