Starbucks is expanding its Mobile Order & Pay initiative to stores in 21 more states, in southern and central U.S., as a pilot effort has proven it be a convenient option for customers.

The mobile order program kicked off as a pilot in Portland, Oregon in December 2014 and then was expanded into the Pacific Northwest several months later, in March 2015, with 650 locations. Currently it's only available to Apple smartphone users, via an app at the App store, but Starbucks intends to deploy an Android version as well, according to a press release.

As of early June, the mobile ordering system is available at 3,400 locations, which is about half of Starbucks' overall store base, states a Starbucks' fact sheet.

The success, according to Starbucks store managers is two-fold: it helps store operators keep lines shorter in the coffee retail space and helps customers get their orders in quicker fashion.

Starbucks store manager Jesse Wenkoff-White first heard about Starbucks new mobile ordering feature when the company launched a test in Portland, Oregon last December. The new feature would allow customers to place an order from their mobile device – skip the line at the register – and pick up their order directly from their barista.

"When I first heard about Mobile Order & Pay in Portland, I was a bit wary," stated Wenkoff-White, who recently underwent training on the new mobile ordering system. "It seemed like a lot of technology." His store is located on the ground floor of a 223-story office tower and the technology aspect is likely not a challenge given the tech-savvy clientele that stop in a few times a day.

"It's fun to have a conversation with customers and teach them how to mobile order," he stated in the announcement. "We can have fairly long lines in the morning and it's such an awesome option for customers looking to run in and out if they're running late. I've had so many people tell me how convenient and easy it is. Our customers that use it absolutely love it."

He said he has noticed that customers seem to be placing the order as they arrive near work, or from work before heading out for a mini-break.

In another location, where the customer base is more family focused, another store manager related that it's proving successful with parents who like to avoid lines with children.