Somebody Please Help Me Out Sim Error

i got a new j210i sonyericsson phone after one week of use when i fixed in my mtn sim it keeps telling me insert correct sim even after i inserted the fomer simcard the fault remains have any body gotten an idea of what could be done to restore the phone to its nomal state?

The phone is locked to a particular network . Thats it.
Throw away the phone , _{Return it to where u bought it only if they will replace it for you}