Denis Doyle/Bloomberg News

AT&T on Sunday doubled the price of upgrading to a new cellphone — a one-time fee that you pay in addition to the cost of the phone itself. The company said it was raising the fee because today’s cellphones are more complex, so the costs of upgrading are higher.

The new upgrade fee is $36, up from $18. Sprint, a smaller wireless carrier, raised its upgrade fee to the same price last year. Verizon Wireless does not charge an upgrade fee for people whose contracts have run their course, though it does charge a one-time $35 activation fee for new customers.

“Wireless devices today are more sophisticated than ever before,” the company said in a statement. “And because of that, the costs associated with upgrading to a new device have increased and is reflected in our new upgrade fee. This fee isn’t unique to AT&T and this is the first time we’re changing it in nearly 10 years.”

But AT&T’s statement raises more questions than it answers. It’s unclear how a customer’s switching to a new and improved phone incurs more costs for a carrier. And the new upgrade fee applies not only to smartphones, but also to traditional cellphones, which aren’t “more sophisticated than ever before.”

“There are the costs we need to recover,” said Mark Siegel, a spokesman for AT&T, in an interview. “There’s time and work involved in upgrading you to a brand-new device. Because those devices are increasingly more complex, the upgrade process is more complex.” He declined to go into more detail about the increased costs.

Without a clear explanation, the increased fee will look to consumers like just another way for the company to make money. Last month AT&T reported selling a record number of iPhones during the fourth quarter of 2011, which helped increase revenues to $32.5 billion from $31.4 billion a year ago. However, it posted a $6.7 billion loss, largely stemming from its breakup fee after a collapsed merger with T-Mobile USA.