GSM calls not going thru randomly on iPhone 4

hi,

my iPhone 4 has started acting up recently. it is not jailbroken, the iOS is 4.1 and i keep my wifi switched on 24x7. i can make calls thru viber / skype, browse the internet, download emails, play games etc. however, at random, i have noticed that when i make a call thru the gsm network, the phone just shows endless calling, the minute counter does not start up at all and there is dead silence on both handset / speaker. the call does not go thru. same is with sms. if i send an sms, the phone just shows endless sending and later on, in messages, it shows sending failed. gsm connection shows the signal but no functionality can be used till i switch the phone off and restart it back up. i have used the phone since august 2010 but nothing like this upto now. this has happened twice in the past ten days.

any advice would be greatly appreciated.

Amer.