Viber problem on iPhone 3G

hi guys i have iPhone 3G with version 3.1.3 i downloaded viber on my iphone but when i open it asks me to put ur number and click continue ...when i put my number and click continue doesnt do anything i know im doing it right cause it founds my country code directly and asking to put my number only ..... i deleted and downloaded it many times still same problem and i tried downloading with iTunes too same again, my brother has 3G S with version 4.1 its working very normal.
can anyone help plz

Thank you