We found 8 possible causes:

1. Flight mode is turned on
2. No credit on your prepaid phone card
3. You've turned on manual network selection and are out of range of the selected network
4. You number has been suspended due to non-payment
5. There may be a temporary network problem
6. There's an error at the receiving end
7. Poor or no network coverage
8. You're abroad and roaming isn't included in your rate plan