hmm......HM Data

 Telecom providers don't make it easy to figure out how they come up with there billing system. Its like looking at your electric bill. Its all in another language.  Cell phone providers seem to try to charge every extra buck possibly, for anything you do that is outside of there calling plan.

HM Data sounds like your son is using the phone for internet to send files, photos, etc..  Cel phone providers can charge about 1.99 per meg of data sent, so the bill can get expensive fast

 A few cell phone plan options to control your bill:

1. call an complain to the billing department. 

Say that there was an error on the bill and that the phone wasn't working.  If that doesn't work, call the customer retention manager that sometimes will remove some of the bill, to keep you on board with the service.  That is if your not locked in to a 2 year agreement, then they know they got you.

2.  buy a prepaid mobile phone. 

This way you can control the mins and services used like a virgin mobile phone or something from target or radio shack.

3.  buy a phone card.

Sure its not as cool as a snazzy phone , but if you pre program your cell phone to make long distance/international calls. you can control your phone bill to call family abroad.  check out www.pingo.com for some decent rates and service.

good luck