We found 10 possible causes:

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