@AM14 - ensure to restart your teen's phone with the Public Mobile sim card inside.
Performing a reset of the device's network settings is a good thing to do when switching providers also.
If you try the SIM card into another phone does the same issue happen; this would identify a device issue.
My guess is, either Chat Features or iMessage is the culprit. Some stuff to review if this is the case.
Both iMessage and Chat Features require a wifi connections or data to send/receive texts.
- if iMessage and you are out of data for your current cycle be sure to have SMS and MMS enabled in the settings of your device.
- if you are using android and you use Chat Features, then I suggest you turn that off, as it requires wifi or data to send/receive texts.
Disable your Chat Features:
- Open Messages
- Tap More options > Settings > Chat features
- Turn Enable chat features off
Something else to consider if an iphone and this is a new number to this phone where a different number could have been used previously...deregister the number on the iphone: https://selfsolve.apple.com/deregister-imessage/.
Set up your phone number On your iPhone:
- Go to Settings > Messages and make sure that iMessage is on. You might need to wait a moment for it to activate.
- Tap Send & Receive. If you see “Use your Apple ID for iMessage,” tap it and sign in with the same Apple ID that you use on your Mac, iPad, and iPod touch.
- In the "You can receive iMessages" section, make sure that your phone number and Apple ID are selected.
- Go to Settings > FaceTime and make sure that your phone number and Apple ID are selected in the "You can be reached by FaceTime at" section.
