Not tech savvy but it sounds like your Mom may have set up a lock with a PIN number on the SIM, you would need to type in the PIN to unlock the SIM to use the phones mobile service. To check this look in settings and search for SIM lock and see if it has been turned on, if it has she would have had to enter a SIM. If she did that and forgot it I have no clue how you would go about getting it unlocked.
Another thing is that Visible did add SIM lock security but it shouldn't have locked you out of the SIM, it is protection for someone hacking you account and transferring your SIM and number to their phone.
