Yes, any time that is not your time is mom's time.
Look dad, you really need to kind of blame yourself on this one. You were pressuring mom to pay for half of the child's cell phone bill, and when she finally blew about it you want to blame her. Now you want to go back to the way it was before, but its probably too late now. If you have been divorced for 6 or 7 years or more you should have realized that if you push things to the point where it blows up, then things get worse.
There is a reasonable chance that this will blow over if you just sit back and give it time. My guess is that mom will realize that she needs to be able to reach the children when they are with you, and will eventually allow the child to have the phone while with you as well.

