I second what N2L says as one option...

but my first instinct was to say ...

"sorry, no, in this house she is not having a phone, if you would like her to have one in your house when she is visiting there, then that is your perogative, but I am not having one for an 11 y/o here..... yes, I am being difficult, but during my contact time I parent her my way, during your contact time you can parent her your way....".

The fact that they don't spend much time with him means she will barely use it anyway, and yes you will seem mean, but.... it's not a mum's job to be nice...

but if that seems too hard... then go with N2l's idea.... it's sneaky manipulative and passive aggressive, and I love it (FX it works of course!!)