Oh man, the echo has been driving me nuts too! Well, not me...just people I'm talking to on the other end of the conversation. It gets so bad that they literally can't hear themselves think, and they tell me they have to hang up.
I swapped out my iPhone over this, and thought it was a hardware problem unique to that particular iPhone. The new iPhone was working fine, but within a few days: echo problem again.
The occurs in a very random fashion. The great majority of the time, there is no problem at all. But occasionally the caller on the other end will complain bitterly about the echo - and that's IF I know them...there have been times when a stranger or businesss associate is on the other end, and they didn't complain out of politeness, and I found out later how annoying the echo was to them. THat sucks.
For a while, I could make the echo somewhat less by pressing the iPhone hard against my ear to sort of "seal" the earphone, and callers DID say the echo got less. This makes me think that the echo issue is an iPhone design problem, NOT an ATT network problem. Plus, I used the Blackjack on ATT for a year and NEVER had an echo complaint.
I have my volume turned up to max, because I find the volume on the iPHone just barely adequate (here's hoping it gets fixed with an update!). However, I have found that by pressing the earphone hard against my ear to "seal" it, it reduces ambient noise and amplifies the volume (this was also pointed out my TinMan in an earlier post). I think I'll try turning down the volume a notch or two as suggested above, and see if that helps.