iphone 4 no service problem

i got an iphone 4 from canada...i live in pakistan ....sender said to me its unlocked n they bought it from apple specially.... but when ive put in the sim it says no service....my phones version is 4.2.1 n modem firmware 3.10.01 and i cntctd the sender n they cntctd apple ,who said tht connect to itunes without sim in it and then insrt sim ....i did it but didnt helped.....what is the problem ???? help plz plz
pentium 4 Windows XP nope