OK so you have Smarty and go to use your phone and get

Not Registered on Network Error

Then you panic now there are a series of issues you can go through not least log into your Smarty account and make sure your phone has not reset any network issues take out the sim rebed it put it back in and ensure you follow the set up guide for a new sim in the control panel of your account. Sometimes sim cards can become unseated if you are rough with your phone!

However, the simplest fix for the vast majority is what the IT Crowd recommend. Switch it off and turn it back on again in other words restart it. Chances are if you are like me you rarely turn it off and just leave it n charge and what can happen is networks can have issues and resolve them or undergo tower maintenance or simply you have a new update pending for your software!

Then make a call this worked for me.

If all else fails then try this guide.
https://www.recovery-android.com/sim/not-registered-on-network-error.html

Hope this helps!