Google has big plans for Hangouts - but it's killing off third party Google Voice apps.

Google Product Manager Nikhyl Singhal shared that news and more on Google+ October 31, shortly after Google unveiled Android 4.4 KitKat and the Nexus 5.

He said the new Android Hangouts app, which now includes SMS and MMS messaging, is "a great step forward," but that Google still has "a lot of work to do" on Google Voice, explaining that the two are not yet compatible.

Part of that work is to "make Google Voice as secure as possible," and to that end Google is shutting off access third party Voice apps, Singhal revealed.

Party's over

"There are a few third-party applications that provide calling and SMS services by making unauthorized use of Google Voice," Singhal wrote.

"These apps violate our Terms of Service and pose a threat to your security, so we're notifying these app developers that they must stop making unauthorized use of Google Voice to run their services and transition users by May 15, 2014."

He also revealed that the updated Google Hangouts app will only work for users with Google Voice phone numbers who have enabled Google Voice on their Sprint phones.

But he added that Google is "working to support SMS messages for all Google Voice phone numbers by early next year," and promised that no one's Voice number will change when that happens.

Voice all over the world

Finally it seems Google is working to get Google Voice up and running outside the US, and working to add Google Voice MMS messaging capabilities. But because it has to work with carriers to do so, it might take a while yet.

"It will take some time before Google Voice is fully integrated into Hangouts, and we appreciate your patience along the way," Singhal wrote.