If you’re the techie type of podcaster, you’d be very open to buying different gadgets to help you in your podcasts.
If you want to record your phone interviews with an exceptional level of quality you should get a handset tap. For example, a great gadget is the QuickTap manufactured by JK Audio. The $60 device will fit in between the phone base and the handset. It has an analog audio input that can be used to attach to a computer. Of course, the limitation of the QuickTap is that it will only work on a traditionally wired phone setup.
Of course, there are other devices available in various stores that will allow you to record phone conversations. Just look at the features and check which ones you will need.