I just found this post doing a yahoo search since I don't know if Verizon will solve this problem.
I started having the echo problem about 2 months ago myself. I called Verizon they sent me out a replacement phone and it does the same thing. They are going to be sending me out another one and I should have it in a few days.
Only thing that seems to temporarily fix it is taking the battery out and putting it back in or muting the phone or switching it to a bluetooh device, seems to fix it for like 2 days. Mine does it where ever I am after about 2-3 days.
I hope Verizon can fix this bug. I wonder if it is anything to do with the firmware 220.127.116.118 update that fixed the bluetooh reboot issue and the getting stuck in 1x. I noticed this issue after I upgraded the firmware. which was also about 2 months ago. It does seem software oriented on the phone itself, too buggy to be the network. Of course my friend updated his bberry as well to 18.104.22.1688 and his doesn't echo here at my house.