Yep, this happens to me as well, iPhone 5 on speakerphone, I can hear other party but they cannot hear me. If I switch back off speaker, all is well. This has been going on ever since I got the phone, and I had great hopes that the recent iOS update (6.1.4) would solve that, but alas, no such luck. We cannot possibly be the only two people with this problem!
No, you're not the only two. I have had speaker problems on my IPhone4 since late last year. Went to Apple store in March and they said was my carrier, AT&T's problem but they swapped out the phone anyway. A carrier problem when I the person I'm talking to can't hear me but I can hear them AND if I switch to the speaker they can hear me. Sometimes I can switch off the speaker and it will work too. Don't think the carrier's networks have gotten that smart yet! But I'm bummed to hear the IPhone5 does it too. And you're right RedpointRik, it's VERY frustrating. Makes me think about an Android phone.