My daughter's SIM card locks sometimes several times a day. What is making this happen? she has an iphone 6, the phone passcode is not locking, just the SIM card. Pretty much negates the reason i got her a phone in the first place
Look for a setting called "sim pin". It will cannot be either under cellular data settings or phone settings. If this setting is turned on, the sim card will lock any time the phone is rebooted or the sim is removed and reinstalled.