viber not working with t-mobile

Hi

I have 2 iPhones, iPhone 4 16 GB and iPhone 4s 16 GB, both using a T-mobile UK monthly plan.

when I use viber app (viber free calling) on data plan I can speak and hear the reciepient for 10 seconds, after that there is no sound, and the call just disconnect, if I use wifi for calling from viber, everything is fine.

I think the problem is with the carrier, but I searched for discussions and troubleshooting from t-mobile website and had no luck.

by the way, the APN setting on my iphone does not show, because it is automatically set by the carrier, however, everuthing else require the use of internet works just fine in the iPhone, except for the Viber problem I pointed out.

perhaps the Viber developers could help me with this problem.

Kind Regards

Yasser