Share your experience!
Sometimes my GSM Connection is broken and an activation is only with a restart possible.
Also fly-mode aktivating and deactivating doesn't works.
I suggest performing a software repair on the device using PC Companion. I have supplied a link to PC Companion that will allow you to perform this task.
I'll try it.
Lets us know how you get on. 
I have the same problem: the GSM connection shows that it is OK (4 bars), but the network type is "Unknown" (for the GSM it is usually "EDGE:2") and I cannot make calls, receive calls and check the balance (via USSD code). This issue has been registered in the GB builds 6.0.B.3.162 and 6.0.B.3.184 and in the ICS build 6.1.B.0.544.
Also in the current build 6.1.B.0.544 the phone is not automatically connected to the known WiFi network. This behavior starts simultaneously with the GSM issue.
I have already repaired the phone with the SUS, but this does not help.
Are these known issues, which will be fixed in the new certified builds? Or should I return the phone to the retail shop and forget about sony?
Hello,
I've exactly the same problem. When I switch to GSM only (because 3G is not available within buildings) I got a very good signal, BUT I can't make a call nor receiving one.
Using SUS doesn't help at all.
I'm living in Germany and using FYVE (Vodafone network) as provider.
Any suggestions ? Download to older Software (and how ?) ?
SW: Android 4.0.4
Modell : ST18i
Kernel : 2.6.32.9-perf
Build : 4.1.B.0.431
Thanks a lot ,
capetown