how to extract telephone audio only from an mp3
There might be much discrepency in the signal's characteristics of the interviewee that depends partialy on the phones they're using, their mobile operator, etc... 
It is much easier to extract the radio's speaker signal. Then, everything that does not come form the radio's host must be from one of the interviewees. 
What you need to do is to analyse the signal of the radio's speaker that might contains more prominent low-pitched components for example (depends obviously on the station you recorded from). Also use a window of analysis sufficiently large to take into account that every stream must last at least 500/1000ms. That might introduce a little stability in your extraction.
If you want to split the different interviewees, you can pose the simple condition that interviewee/host must alternate.