Q: I have 2 phones on my plan. If someone is using the Apple ID from a completely different phone/account does it use my data or theirs?

If it's using the same account as yours then it is using your data. But if she is logged into another person's account that does not use any of your data.

The Apple ID should be irrelevant.

Your cellular phone data is billed based on the device that the SIM is inserted into unless you have some kind of plan that also bundles in Wifi data from certain public access points, in which case her phone may use your wifi passwords to consume data (iCloud can sync those settings too). If you have no SIM the data usage is from the device that the plan is registered.

If you are referring to just 'cellular data' then the issue lies with your son & it is his own data usage habits.

Frankly if your son is old enough to have girlfriends he should also have his own Apple ID. Sharing them can lead to horrid situations, especially if he hands out passwords like this. I think the minimum age for Apple ID's was 13, but that may have changed, so check if he is younger than that.

It is possible that she is compounding his data usage problem, if for example she downloads apps, music, podcasts to her device. His device may also download them too since iCloud can 'automatically download purchases' & other con ten, if that happens on cellular data it will go onto his plan. You can specify what apps can use cellular data in the Settings app, also review his data usage in that app to see where his recent activity is using data.

About cellular data settings and usage on your iPhone and iPad - Apple Support

P.S.

If this forum is using the same Apple ID, she may be getting your emails too depending on what she has setup with your/his Apple ID.

Apple ID has nothing to do with cellular data usage.

Your son should start changing all his passwords immediately, or she'll continue to have access to all of his private information and accounts.

For additional information you can check here https://appleid.apple.com/faq

Can I share an Apple ID with someone else?

Your Apple ID should not be shared with anyone else. It provides access to personal information including contacts, photos, device backups, and more. Sharing your Apple ID with someone else means you are giving them access to all your personal content and may lead to confusion over who actually owns the account. To share iTunes & App Store purchases, photos, a calendar, and more with someone else, try Family Sharing, iCloud Photo Sharing, or other easy-to-use sharing features.

How can I keep my Apple ID secure?

Security and privacy are very important to Apple and we provide a number of ways to secure your Apple ID and protect your privacy including strong passwords, two-factor authentication, and more. Learn more about security and your Apple ID.

Rysz wrote:

 

Apple ID has nothing to do with cellular data usage.

 

Your son should start changing all his passwords immediately, or she'll continue to have access to all of his private information and accounts.

This is exactly right.

Cellular data usage is a feature of the SIM & the phone contract. She could only use his data if she physically uses his SIM card in her device or he allows others to use his phone as a wifi hotspot.

I'm afraid your son is responsible for the increased data usage, unless he's handing the phone to others (which is his responsibility anyway).

I realise it is complex when we mention several possibilities but iCloud cannot do what theratter said earlier, I think it was a misunderstanding.

iMessage can send from multiple addresses & telephone numbers, sometimes it will depend on if you have an internet connection. If you have no internet it can use SMS (old style texts shown in green on iOS), which is only sent over the cellular network. It may explain how she sent messages from her number & his.

In the conversation tap 'details' and then tap the Info '' - the sending address will be highlighted blue. You may find her messages come from an email & a phone number. It can be tough to work out which one will be used if you use multiple Apple ID's.

Untying this mess could be difficult, in theory resetting his password will lock her out provided she has not added herself to his password reset details.

I wish you all the best with this situation, it seems tricky to say the least.