LG GX 200
INCOMING VALL DISPLAY

I M USING LG GX 200... I JUST SWITCHED OVER FROM PREPAID TO POSTPAID (LOOP SERVICE PROVIDER)..WHENEVER I GET AN INCOMING CALL IT DISPALYS AS AN UNKNOWN NUMBER... EVEN THOUGH I HAVE ALL THOSE CONTACTS IN MY PHONE MEMORY SAVED... I CANNOT EBEN CSLL BACK ON THOSE NUMBER?? PLEASE HELP