I am a parent, but of a 20 month old, although my principle method of parenting will never change, it's one thing I'm absolutely firm on, and it sounds like you are too. All that changes as children age is HOW they tantrum. My "2 year old" kicking off in the supermarket tonight because I stopped him trying to open a packet of pancakes, is the exact same childish behaviour you'd see if you tried to take the smartphone away, just manifested in a different form.
My wife asked me tonight on the way out of the supermarket doors "when will this screaming and tantrumming stop?", I answered, only half joking "when he's about 35".
Obviously, in this situation, close family are taking on the parenting role. It does require co-ordination between all involved adults and I can understand how you may feel snubbed that her new number was not communicated to you. With kids though, we need to remember not to take things too personally, and, if the adults around here who knew of the change of number were older, then that could be understandable too as they would likely not see it as important.
A lot of this depends on her maturity, not her actual age. 11 can mean a child who's essentially brought up siblings as an "adult", or it can be an 11 who's led a sheltered life and has no idea of the outside world at all. It's very difficult to generalise.
What would you prefer to happen as of now if you had an unrestrained choice? Her to lose the smartphone completely, be educated in its use, or something else?