For legal and compliance issues, at least here in the US, attaching equipment to the POTS system requires the use of an approved DAA (data access arrangement) module. This protects the POTS from 'foreign' voltages and grounds. These usually also handle the voltage translation needed to interface to logic systems like the Arduino.
Thank you for the info, I can see that the question was a bit vague, so let me start over.
I would like to use an old telephone as an input / output device connected to an Arduino board, I am not going to connect to the telephone system.
My question is, how do I connect the phone to the Arduino, in a way that when I speak into phone I get an analogue signal (at a low voltage) input to the Arduino? And visa versa (analogue output to phone speaker)?