Siemens SX56 signal and sim problems

I am having a problem using my Siemens Sx56 Pda phone.
I am right now in Pakistan. I purchased SX56 from my friend but it was
having a code on it. I show it to a mobile seller here in Pakistan.
He break the code by installing new windows on it. But after 2 weeks
mobile got problems. Sim was already inserted in the SX56 but when i
put On SX56 it gave message that Insert sim and enter Pin . Some time
it recieve signals but when I place call or recieve any call it
disconnect the call and leave signals and again gave same message
that Insert sim and enter Pin.Therefore You are requested to let me
know what is the problem with it and what should I do to solve this
problem .
Because I am unable to place or recieve any call

Your early and favourable reply in this regard will be highly
appreciated.

Thanks
Yours Truly
M.Mahtab
Direct # 0092-333-8114973
Email : mahtab20@pakistani.com

Dear Qman SIM CARD is fully seated and the black rubber door is also closed. but getting the same problem please help me in this regard. i m very tensed about it

Dear Qman. right now i m out of city and SX56 is at home.
I dont remember exactly i think ROM version is 2.1 and radio band is running on 900 coz here in Pakistan Gsm work on 900 band.
what u think what will be the problem in it ? plz help me plz plz