Although the carriers might not want you to play with it, many cellphones on the market today have at least fledgling compatibility with VoIP protocols.  Significantly absent from the Android OS, however, are the SIP and IMS stacks that facilitate IP-based call initiation and quality-of-service management.  It’s an omission that has led some to question Google’s intent toward non-traditional communication on Android-powered handsets, and others to suggest that the future of VoIP standards is not so tied-up as some of the companies already using SIP/IMS might claim.

“Maybe Google wants to replace SIP and IMS — or subsume them within Android. By leaving these standards out, Google can position Android as the default platform for mobile devices and once it gets its fingers around the carriers from the device side, it can then take over the ecosystems for SIP and IMS and move the service providers onto these systems — or whatever technology Google positions in their place” Stephen Wellman, InformationWeek

One possibility is that it could dovetail into the persistent rumors of Google acquiring popular VoIP platform Skype, which has always used a closed ecosystem rather than a more open, publicised alternative such as SIP.  Of course, that would seemingly run contrary to the “open access” ethos behind Android and the OHA, but by omitting a defined VoIP technology they could argue that developers are free to load whichever of the many third-party SIP/IMS stacks onto their handset as they so choose.