We found 10 possible causes:

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