Do you think It's okay for my mom to do this?

[Andy]

I'm not sure about the law out of the UK but in this county- UK, anything you have bought for yourself or have had bought for you and given to you is considered your property and yours to do what you want with. She CAN'T confiscate it, it's yours.

So:

Step 1) Announce to your mum that you want it back immediately and she will have no access to it, that's not negotiable.
Step 2) If she says her house her rules and sort of does that threatening to kick you out thing... Well then she's just put her foot in it hasn't she? Because if she truly cared about your well being she wouldn't want a 16 year old girl homeless now would she? - Make sure you bring that up, how ridiculous her argument is 'I want to see all your messages because I care about you, but if you don't let me see them I will happily kick you out and watch you roam the streets' .... just WOW.... what a ridiculous notion.
Step 3) If she still threatens to kick you out, either a) leave or b) if you dont want to leave, tell your dad to take the phone back, and tell your mum 'I'm not negotiating with you, your arguments are ridiculous and transparent, this conversation is over. Give dad the phone back or give it to me so I can give it to him.'