We found 12 possible causes:

1. There's no credit on your prepaid phone card
2. Your number has been suspended
3. Flight mode is turned on
4. Fixed dialing is turned on
5. Your mobile phone is overloaded for unknown reasons
6. Your SIM is faulty
7. Not adequate network coverage
8. There's a temporary network problem
9. There's an error at the receiving end
10. The selected network is out of range
11. The selected network mode is not available
12. Roaming isn't activated in your rate plan