GT-SIM trouble

[Przemolog]

Originally Posted by GSM_student

Sorry, could I know what meand USSD code? What I can do with it? Thanks

That's simple. This code "wraps" the number you want to dial and triggers the callback (in case when the phone's software can't do that).
E.g. if you want to call +1 234 456 7890
dial
*125*1*12344567890#
instead of
+12344567890
Tha's all.