I share your basic feelings about kids and technology. That said, all 3 of our older kids have cell phones!
DH gave DSS a super-basic one when he was only 6, for staying in contact with whichever parent he was away from. (Super-high-conflict divorce, Mom lives across the country...) As our kids got older; it became more common for their friends to have cell phones; and DSS's mom upgraded his to include internet, it seemed increasingly unfair that my (older) sons didn't have phones. We started by getting them a basic one, to share, when they entered middle school. By high school, we gave them their own. It really is convenient, to be able to contact them, if a practice is cancelled, or they stay after school for a study session. And AT&T has a very affordable unltd. internet access fee, if you have "dumb" phones - so that's the only kind we have.
Still, I frequently take the kids' phones and put them away for a while, when it seems like they just can't get off of them (or if they go straight from computer to TV to texting). They need to do other things and they need to interact with the people they're with. It irritates them when I take their phones, but they understand why I do it. It's not a punishment, but a redirection. They know when they moderate their own technological behavior (and can spend time and have conversations with their family members, without constant texting or web surfing), no one will moderate their behavior FOR them, by taking their phones. It's a learning process.
Be careful, since the phone is from your DSD's mom. If Mom expects to be able to talk to or text her on it, it might seem hostile for you guys to outright ban the use of it, in your house. (Even though I realize hostility is not your chief objection to it.) If you object to kids using them in your home, you could at least put it in a place where, if she wants to call her mom or check for and return messages, she can.
But I have to say, it is much nicer when divorced parents consult and are considerate of each other, in matters like this, instead of taking advantage of every opportunity to be the "cool" parent, by providing whatever the other parent won't buy!