"As Microsoft's strategy is to grow Lync sales by selling into
the Microsoft Office installed base, the first alternative you
should consider is Google Apps, a direct cloud competitor of
Microsoft's Office 365. Although Google doesn't yet offer a PBX,
OnState Communications offers a cloud-based PBX in the Google Apps
Marketplace. It also stands to reason that Google will add some
degree of PBX capabilities to Google Apps.
"Twilio, a self-described cloud communications vendor, offers a
platform to build voice and SMS applications using simple APIs.
Twilio also offers an open source phone system through its OpenVBX
offering. Twilio is targeted at developers, whereas Lync is a
ready-to-use platform for companies. However, systems integrators
or managed service providers could take the Twilio APIs and build a
repeatable solution that offers much of Lync's capabilities."