I have a problem concerning the limit call time in Blackberry?

Hi,
I have a BlackBerry phone that is GPRS and 3G capable. My problem is, when I am using it on calls for less than two hours, it will disconnect with an error that says, "Call Failed". I am under T-Mobile and it always end a call with GPRS connection on top of the screen. It seems that my phone does not allow me to do things at the same time as it always disconnect. Any ideas about my problem?
Thanks.
