Problem in 3G mode but it runs fine in Edge mode?

Hello,

I have fallen my iPhone 3GS and since this time it runs badly in 3G mode.

If my iPhone is in Edge mode, data and telephony are ok, but if I switch to 3G mode, I lost the signal.

Wifi and GPS runs always correctly (in Edge or 3G mode).

Do you have any ideas where is from the problem?

Thanks a lot for your help :)

Chris