What model phone?
Did this just start happening at random?
Is it a T-Mobile phone and SIM?
Are you a customer of T-Mobile?
Did this phone recently update or does it need an update?
Have you removed and reinserted the SIM?
Do you need a new SIM?
Has the phone been active on your account before?
Has it been reported lost or stolen?
Is it blacklisted for non-payment?
Are you able to select an operator from the Network settings menu?
Are the APN settings correct?