Major enterprises like Starbucks and Walmart have tens of thousands of employees out interacting with customers, and they are largely cut off from general corporate communications.

Cotap CEO Jim Patterson has made it his mission to bring them into the fold.

Formerly head of product at Yammer, Patterson left after the Microsoft acquisition to start Cotap, a mobile messaging service for businesses. The San Francisco-based company released a free iOS app in October that employees from more than 6,000 companies now use. Once the workers plug in their corporate email, it adds them to a shared directory, where they can connect and communicate with their fellow workers.

“We want to be the fastest and easiest way to text message a coworker,” said Patternson in a conversation with VentureBeat.

The company is planning to launch a premium version of the app around March. That’ll include the capability to add people without a corporate email address and send out push notifications to all (or a select group) of employees. Cotap hasn’t announced pricing yet.

Workers have hundreds of ways to communicate, with enterprise-focused efforts like Yammer and BlackBerry Enterprise IM or consumer-oriented services like Gchat and Skype. Patterson thinks Cotap is a great mix of ease of use and security compliance.

Cotap today announced a $10 million round of financing that’ll enable the company to roughly double its 17-person team by the end of 2014. It’s placing a major focus on engineering, with an Android app due out next month and features like cloud storage file attachments in the works, though it’ll also expand its sales and marketing efforts.

Eventually, Cotap aims to bring its messaging service to every device that people may use to work. It’s targeting all sorts of businesses, but Patterson noted that there’s a lot of value for companies with a lot of “front line” employees: retail, hospitality, food and beverage, and so on.

“Now all of those employees are coming into work with a computer in their pocket,” said Patternson. “We really think there’s tremendous value to be unlocked there for companies who really embrace this. … Our company mission is to make every worker a knowledge worker.”

Following Cotap’s $5.5 million round from last May, this new financing brings the company’s total funding to $15.5 million. Existing investor Emergence Capital Partners led the round, which also included participation from Charles River Ventures.