Viber messenger not working on my ad-hoc wireless network

I have an iPhone 5 (iOS 6.1.1) and I installed Viber messenger (version 2.2.2.86) which enables you to make phone calls over the internet. The messenger works perfectly when I use a wireless router to connect to the internet (which I don't have at home) but when I connect to my Windows XP ad-hoc network, I can't make phone calls. the call is dropped after 3 to 4 seconds and I can't receive calls either. This only happens on my ad-hoc network. - The computer connects to the internet through 3G modem - I tried updating to the latest version of Viber (2.3.1) from apple but no good - My friend has an older iPhone (probably 3G or 4) and he has the same exact version of Viber. When he tries to use my ad-hoc network, it works and he can call and receive calls. - I can't use my 3G sim card directly on my iPhone, I have to go through the ad-hoc network. What could be the problem and solution to this dilemma?