For a regular phone, there is recording equipment out there to connect to the phone and then download the recording later. Here's a great article with multiple recommendations for recording hardware and software compared for quality and cost.
I like to use my Skype Voice Over Internet Account on my computer for these calls. I put on the headset, call the interviewee and then hit the software recording button to get both sides of the entire phone conversation. There are two low cost recorder applications designed to work well with Skype. They are HotRecorder and Pamela for Skype. I prefer Pamela for quality and ease-of-use reasons.
Once the call is recorded, I just load it into my audio editing software for volume and other editing, then tack on my intro and exit segments for podcast upload.
Want more info? Here's our Podcast Tutorial & Resources.

