Best answer by terrywebbs
If you have recently ported over an old number or inserted a new SIM into a new mobile?
Best thing to do is restart the mobile by turning it off and back on again.
If that doesn't work check you have enough call credit and allowance.
Check you have the latest software installed iPhones are best updated with itunes.
Other mobiles can be updated from the settings/about mobile/update.
Best to have wifi enabled this is faster.
Next thing is to check the mobile is unlocked for the ID Mobile network.
Do you have a signal strength meter and has the SIM been accepted by the mobile?