I have a similar problem.
I can't do phone calls longer than 10 minutes. Phone drop the call. It shows the icon that SIM card is missing for a short time. Call drops in various tiems. Iimmaediately or after 1-2 mins or 5-8 mins. Strange but if I call the operator's customer support number for testing - because that number is free - usually call drops after 9:59 exactly. While call to other numbers call drops after various times but never reach 9:59. Even in 2G or 3G. Indoor or outdoor. I tried three SIM cards from various providers. Tried reflash the phone but doesn't helped.
I call the operator, what if the phone is stolen phone (my phone is a second hand unit) and the owner disable it by the IMEI number. But operator said, in this case phone can't login to network.
Official Nokia servie can't help me. They ask for the IMEI then the pepole said this unit is from the USA and they can't repair. Then I go to the 'GSM shop' now I waiting.