Phone connection breaks while going from 3G to GSM iPhone

Hi !
I do have the iPhone 4 GSM (software 4.3.1. with t-mobile network + bumper) and problems with loosing the phone call when moving from an area with 3G service to GSM (for example going into an elevator or to the basement while having a call). GSM coverage is 100% in the the elevator or basement, although the iPhone looses the phone call while switching from 3G to GSM. It seems the transition is not smooth. My Blackberry does not have this problem. My workaround is to disable 3G.
Do you guys have the same issues ?
Best Andreas
iPhone 4 GSM iOS 4