2100 sim registration failed?

[iecep]

I had a prepaid 2100 phone which always display simcard registration failed when i insert a good simcard by network operator numb1(900mhz),fortunately the phone operate ok when i change simcard using operator number2(1800mhz).can anybody explain what should be the problem of my phone.This is the first time i encounter such flaw?thanks in advance.

[gre_soul]

sounds like its barred to me!