[Jim727]

Does the phone have call forwarding?

if so, get the kid an identical looking phone with a different provider where you pay the bills. Put the suspicious phone in a drawer somewhere with calls on forward to the phone you bought. You'll get a record of the calls made, the phone will always be in the same place, and mom can still see/call what she thinks is "her" phone. You might need to make the tracked phone mobile, but that's a detail you can work out.