I had an old Sanyo on Sprint. Call quality on the iPhone is good. Coverage is good, and I've taken it on travel (NH, Florida). I like that I can make conference calls from this phone - call one person, conference another into the call.
It apparently works well as a speaker phone. People on the other end hear me fine, and I hear them fine too over the speaker.
It works fine with the ear buds/microphone that comes with it. I've done lots of calls that way, and it works well. I've also used the optional microphone with the Bose QC2 headphones, and that works equally well.
Ultimately it's a cell phone. Check the sales terms. When I bought mine, I had 30 days to return the phone and cancel the AT&T contract w/o penalty. I don't recall if there would have been a restocking fee for the phone. Check. Point is, you should be able to return the phone and cancel the service if it doesn't work well for you.
Hope this helps!