Not registered on network

I've seen this error several times with my customers and these are some of the possibilities.

1. Did you change service provider recently, such as from Verizon to AT&T? If that's the case, then it means that the phone is not unlocked and can only be used with a specific provider.
2. It could also mean that the SIM card has malfunctioned, and needs to be replaced with another one.
3. Lastly, maybe the service on the phone came to an end (prepaid service and didn't add funds, services were cancelled.) With phones from SafeLink, Assurance Wireless and others; if you don't use your government provided service at least for one call and one text a month, then the service get's disconnected, and that's when you may see that error.
4. Hope this helps.