What does "network not registered" mean?

[AC Question]

I can't make calls or txt on my s4 since a few days ago. Every Sim I insert it tells me "network not registered... what does this mean? And please what do I do?

 

[mhinc]

I can't make calls or txt on my s4 since a few days ago. Every Sim I insert it tells me "network not registered... what does this mean? And please what do I do?

Typically it means one of three things

1. You are trying to enter a SIM card from another provider and your phone is not unlocked.
2. If your phone is unlocked, and you have used multiple sim cards, it could be a hardware issue with your SIM slot.
3. The SIM cards are simply not registered anymore on whatever provider they are with.