If I'm doing an interview over my iPhone, I use an app called TapeACall. I often call into media calls that can be very long and it works well. I just plug in my headphones and wait for my turn to ask a question, rather than holding my phone by my ear for 15-20 minutes.
I'm not sure what you're going to be using the audio for, but if it's for broadcast purposes, that's your best option.
If it's just for transcribing, I just use an Olympic digital recorder and put my phone on speaker. It allows me multitask while on a conference call. Then I take the audio and use this to transcribe quickly:
http://transcribe.wreally.com/
I also use my digital recorder for in-person one-on-one interviews or media scrums. I find the quality much better than the voice recorder on my iPhone.