http://roaminghack.blogspot.com/
There is a very simple way to do this. Become unprofitable.
If you're paying them $50 a month but costing them a few hundred - They most definitely will cancel you.
So how is it done? A couple of years ago you might of noticed that everyone stopped paying roaming fees. One wireless giant started the "no more roaming charges" program, and the rest followed like the sheep that they are. But just because you are no longer paying the $0.69/min doesn't mean your provider gets off scott free.
Even though you stopped paying for roaming charges years ago, your wireless service provider has been eating the costs. This is done to keep complaints to a minimum, and of course, you as their customer.
So let's do the math.
At $0.69 a minute, a 2 hour long call in a roaming spot just cost your carrier almost $83.00
Even with some of the higher end plans, this would still be quite a hit. But would it be enough to cancel you?