I own a first batch FP1, which I hold dear but since a few weeks I have following issue. The phone is showing full signal, recognizing both SIM cards in the slots. I currently use a Vodafone (D2) and Telekom (D1) carrier SIM. After rebooting the phone I can usually place a call to a number or to my mailbox. However usually right after the call I cannot place any calls or even receive a call/SMS. The caller screen comes up (where you can hang up) but there is absolutely no sound to hear; after a few seconds (~10s) the call is automatically hung-up (red hang up button turns grey) and the caller screen closes. Data connection is similar - sometimes it works, but when I can’t place a call I also cannot use data connection or SMS. After a while (sometimes an hour) I received a call again…
SIM cards shouldn’t cause trouble: I successfully tested both SIM cards in a second phone.
I did a factory wipe, reset the OS from SD card to no different picture: no apps installed at all but same behaviour. The phone seems to be registered, I tried to register each SIM to a forbidden provider (failed) and registered it again to the designated provider (success). Again the same behaviour, sometimes I can place a call, sometimes not. I disassembled the phone, cleaned and reconnected all cables. The signal strength changes depending on my location.
Please help, I am out of any further idea - it would turn my phone to a spare part