If by phone you mean POTS ("plain old telephones"), then get an old headphone plug, an old telephone plug, and splice the middle two contacts (red and green wire) of the telephone plug into the headphone plug. Then just run that into the Line In jack on any sound card, and hit Record in your favourite audio recording software. Just make sure to plug the adapter in after you start the phone call and to unplug it right after or Bad Things will happen (sorry, I've forgotten exactly what - something about it picking up the phone line and leaving it, I think).

--- Mr. DOS

My suggestion is for you to use Skype. You can then buy a little bit of skype credit or a skype subscription (very very cheap and unlimited free calls to landlines), call the number as you would on a normal phone, and use an extension for skype that allows you to record the conversation. You would get the best audio quality with skype-to-skype phone call though for podcasts, and better than hacking into your telephone when calling someone on a normal phone.

I would be a bit careful plugging a sound card into the telephone circuit. The ringing voltage is around 100Vac, which you won't want. I don't know what voltage you might have on the line in if you followed Mr. DOS's method - in use it might be below 10V dc.