Go to My T-Mobile and see if your family plan says "Included
WAP Access". Technically all plans come with free
WAP access, and the $4.95 T-Zones thing is for access to the (crappy) T-Zones homepage. Rather than charging you for
wap access if you don't have it, they'd probably just deny you access.
Bottom line -- you should be JUST FINE. No charges should show on your dad's bill, unless you're downloading PAID ringtones, those are 99c each.