Thread: HTC P3450 Network Connection Problem

I am having a problem with my HTC Touch P3450 phone for the past 2 weeks. Although the GSM signal looks good, I am not able to make/receive calls. I am on "Phone" mode (not Flight Mode or similar). Everything else (WiFi, programs etc.) works fine.
My operator is Turkcell and I live in Istanbul, Turkey. The phone was purchased from the United States.

I have done a ROM update to ROM version 1.17.707.2B WWE but the problem still exists.

Has anyone else experienced this kind of problem? I suspect that this issue has something to do with the 3G upgrading process going on in Turkey.