SIM not working! Weird stuff!

[Giganizer300PRO]

I have 3 phones and 2 SIMs.

I have my regular phone and regular SIM, then, there's my mom's SIM and my mom's phones.

My phone is a PrestigioMultiShitPhone, my mom's phone are an HTC Desire X and a Galaxy Core (no, not duos and not Core 2).

Every SIM works in every phone, except for my mom's SIM not working in the Core, but my SIM does work there.

The Core has a corrupted efs problem, but a GO Launcher EX is installed.

When my mom's SIM is in the Core, it all looks fine, but when an attempt is made to use it (a call for ex.), an error message appears, saying "Not registered on network". When a call is made to that SIM (in the Core), it says that the number is unavailable.

There are full bars on all.

I don't know what to do.

Please help. Thank you for your effort!

[Giganizer300PRO]

If your SIM works in the Core then your mom's must be failing. Consider having it changed.

The SIM is a brand new, it was changed today, but same issues were present with the old one.

[Giganizer300PRO]

it might not have been activated yet, just a guess.

I thought of that too, but the old SIM was working just fine in other phones, but had the same issue here (in the Core).