If that does not work and some of the Samsung users don't have any suggestions I would suggest you contact a mod here
Also by any chance is the phone in Suspend status....which I doubt.
lol...I can't do that for you, I am in BC. Just another thing...if you contact a MOD you will need to know your Mom's information...phone #, account, email address etc.
When you are with the phone call 611 and see what it says....if sim removal doesn't help and the account is active it may be a similar issue to this members....
Bring your mobile and swap SIM cards between devices and try to call someone (other than each other) to see what occurs.
This way you'll be able to determine whether the problem is with the mobile device or with the account (SIM).