Consumers can now make almost any sort of transaction from their mobile phones -- except buy a burger and fries.

Fast food has routinely lagged behind other industries in tapping the ever-present smartphone, with moves largely limited to paid ads and apps with store locators and nutritional information. But that's starting to change.

McDonald's, the biggest chain by sales, two weeks ago confirmed it was testing mobile payments in Salt Lake City and Austin, Texas. (It's been testing mobile payments overseas for a while.) The U.S. test includes mobile ordering, with customers able to pick up food in stores, curbside or at the drive-thru, as well as promotions and a loyalty program. KFC, Burger King and Wendy's are also experimenting with mobile ordering on limited bases, and Jamba Juice is working to broaden the availability of its mobile-payment app.

There are a few successful pioneers: Domino's has brought its Pizza Tracker, which lets users order and follow their pizza's progress, to the phone; Chipotle offers a popular ordering app; and Starbucks is leading the way in mobile payments and loyalty programs.

Mobile is the fastest-growing part of Domino's digital business, said Dennis Maloney, VP-multimedia marketing.

About 35% of the pizza chain's orders come in via digital channels, with one-third of those digital orders coming from mobile. That's up from 23% a year ago.

"Most people are looking at mobile now as a core component of any commerce business," he said.

So why isn't mobile ordering and buying more widespread among other major restaurant chains? For starters, their size complicates things.

"Trying to roll something out on a national scale is difficult," said Dirk Rients, senior VP and director-mobile at Omnicom's DDB, Chicago. "We're starting to see brands explore and pilot ideas like payments and ordering in small markets before slowly moving it out to larger markets."

Digital executives see mobile as more than just a payment platform and suggest smart fast-food marketers will make it the cornerstone of a rich loyalty program.

"The big thing fast food needs to realize is that mobile is so much more than a promotion and media channel," said Sam Cannon, executive creative director at Razorfish in Chicago. "Of all industries, [quick-service restaurants] have a unique opportunity to use mobile as a platform for deeper recurring interaction between consumers and the brand."