Eseries / Communicators :: Cannot Make Or Receive Calls On Nokia E52

I cannot make or receive calls and messages on my nokia e52 sometimes. When trying to make a call it shows a small lock in locked position near the green phone symbol. I then restart the phone and it works fine. It only occurs rarely like once in a week or two and you wont know unless you try to make a call or some one calls your land phone and tells that the mobile is switched off. I am using the latest software and has tried the factory reset.