Not holding MSID

[s2000gtx]

hello guys my ppc6700 is not holding the MSID so when ever someone callsme my cellhpone doesnt ring. the thing is i can call them and talk to them also i can send text messenges too. the thing is i can make and do outgoing calls and text messages but i cannot recieve any imcoming calls or messages. and everytime i put the msid as my phone number as soon as im finish it goes back to 000000000. Please what should i do