Either look online for "unlock p900" for instructions on how to do it yourself or take it to an independent mobile phone shop who should do it for a tenner at most.
You have to unlock your mobile before using the other country SIM .This can be easily unlocked via codes.If you need an unlocking service,simply get it from http://www.theunlocka...ricsson-p900/rs13wp9/ .Here i found the valid codes.Through this you can unlock your mobile easily.