AT&T and Verizon Wireless now charge $20 fees when customers upgrade their mobile devices, with one notable difference: AT&T charges the fee even when customers bring their own phones to the network instead of buying devices from the carrier.

Verizon started charging a $20 upgrade fee on Monday this week, matching the $20 fee it introduced in November for customers activating new lines. MacRumors published a leaked memo, which said the upgrade fee applies when customers buy their phones from Verizon—whether they pay full retail price or make monthly payments on an installment plan—and also when customers enroll in Apple's iPhone upgrade program. Verizon won't charge an upgrade fee when customers supply their own equipment. (A $40 upgrade fee still applies when people get an old-fashioned two-year contract.)

Verizon's memo compared its own upgrade fees to those of other carriers, noting that T-Mobile doesn't charge a fee, while Sprint charges $30 and AT&T was charging $15.

But that's already outdated, as yesterday Droid-Life noticed that AT&T boosted its upgrade fee from $15 to $20. AT&T started charging the $15 fee in August of last year and has now raised it just eight months later. AT&T's website says customers now must pay $20 when they activate a new phone whether they bring their own devices or are on one of AT&T's monthly installment plans.

This is cheaper than activation fees for two-year contracts, which AT&T sets at $45. But even with carriers shifting away from contracts, the latest moves from Verizon and AT&T show they're not going to let device activation fees fall by the wayside.

While AT&T's webpage on the fees doesn't provide any justification, Verizon's memo said it is instituting the $20 charge "to cover our increasing support costs associated with customers switching their devices."