We found 9 possible causes:

1. You number has been suspended due to non-payment
2. You've turned on manual network selection and are out of range of the selected network
3. Flight mode is turned on
4. No credit on your prepaid phone card
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
9. Your mobile phone is set to connect to 4G networks only