AT&T has added Microsoft Lync to its unified communications portfolio, allowing business customers to communicate over networks with instant messaging, voice-over-Internet calling, and online meetings.

The largest U.S. cellular network, with more than 110 million subscribers, offers a range of business features for startups, small to medium sized businesses, and larger enterprises alike, all over its cellular and wireline network.

Businesses will now have a complete unified communications solution, the cellular giant said in a press release today, that also allows a wide-range of tablets, smartphones and other devices over a bevy of different platforms, not limited to Windows.

Microsoft told ZDNet in an emailed statement that AT&T business users can choose to run Lync on-premise or as part of its Office 365 cloud-based services.

In prepared comments, Microsoft's Derek Burney said: "This expanded relationship with AT&T underscores the growing role Lync has in the Unified Communications landscape and is an opportunity for both of our companies to continue to deliver businesses the tools they need to collaborate."

AT&T also said that it plans to open up its range of APIs to users of Lync to allow developers to tap into the platform to create custom mobile and Web-based business applications.

Lync will become increasingly important in Microsoft's portfolio of Web-oriented products as the enterprise platform will begin to share with Skype , which was acquired in 2011 for $8.5 billion.

The software giant recently released a version of its Lync client to iOS devices, for both iPhone and iPad , earlier this month.

The AT&T-Microsoft move will likely help boost Lync sales, which recently said it had 5 million customers on its most expensive version, according to Bloomberg.

Updated at 12:10 p.m. ET: with additional details from Microsoft.