Hi!
I have two Droid pro phones from Verizon.
They are both unlocked to use abroad and have latest system update - 4.6.8.XT610.Verizon.en.US.
Baseband version: N_01.81.00R.
I am using phones in Kazakhstan on KCell (Kazakhstan local telecom provider) which operates on 900/2100 frequencies.
And I see very strange behaviour with this phones:
While the phone on GSM frequency I can't hear anything - absolute silence during a call while on GSM, or I can hear for a first few seconds (1-3) and then again - nothing.
I can hear only signals from numbers on Dialpad when I press it. 
At the same time, if the phone switched to UMTS band (I see that because 3G icon is shown) I can talk easily, no problem at all. 
Tested on both phones - same situation.
Tested on other providers (Beeline) - same.
Can anyone can tell, what all this mean and how can I solve this?
Does anyone experienced similar problem?
Thank you!