We found 8 possible causes:

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