Rollover Gotcha
by Tom Temple
7 January 2010
If your wireless usage is between 550 and 2100 min/mo and you’re on AT&T, it might strike you as a good idea to get the the 2100 min/mo plan to take advantage of rollover minutes. That is because you don’t have to worry about going negative and paying the ridiculous premium, while at the same time taking advantage on the price non-linearity.
Let’s just remove months and talk about minutes per dollar. From 550 you can buy 150 more minutes for 15min/$, 850 for 28min/$ or 1550 for 31min/$. Keep in mind that minutes don’t roll over for more than a year.
Given a probability distribution over monthly usage, you’re faced with a well-studied optimization problem from inventory management. The solution is basically to find a safety level below which you get the 2100 and above which you get the 550.
That is, until you try to downgrade your plan and they tell you that you lose your roll-over minutes when you switch. Fuck!
