We're recently just set this up. Our studio telephone number is a VoIP line and we connect to it using a SIP server. You can simply download a free VoIP Softphone (or similar) and feed the line in of the soundcard from your mixer output. The output from the soundcard can be fed into a spare channel on your mixing desk. Works quite well. A lot better than having to pay hundreds of pounds for a telephone balance unit
Depending on the quality of your PC soundcard will depend on the quality of the calls - we found that oboard sound cards were not good at all - but a cheap second hand Creative or Soundblaster (or compatible) bought second hand from an auction style website worked a treat.