Recording interviews over the telephone is powerful tool for creating content and info products.
However, be aware of headsets and telephone hand helds.
The diameter of the mouth piece (microphone) will very the quality of the spoken word delivered via the telephone line. I will not get into the technical aspect of KHz and bandwidth, but the thing to remember is:
1) The quality of the telephone. Those $9.99 telephones you can buy at the pharmacy or retail sales stores are... well.. worth 10 bucks, so be sure to use a quality telephone
2) The smaller the microphone diameter (the size of the microphone capsule in the telephone) is in most cases limiting or enhancing the audio quality. The larger the capsule usually gives you better low end (bass) response and consequently a better voice recording
3) Wireless phones and cel phones... you are taking a chance that interference, battery decay and signal strength will effect your recording.
So yes, size does matter