For North America there are ANSI T1 standards, TIA standards and CSA standards providing answers. In addition, the FCC and the CRTC limit how much current you can draw from the line. Those regulatory requirements are incorporated in the equipment standards. Since PSTN incorporates everything from old analog lines to new ATM switches, you'll have to be more precise.
I believe that you find most of your answers for Europe in what used to be called NET5 from ETSI. It would even include the UK. By Golly, there are Rules out there. Ciao, Vic
