For example, Squad (the standalone game from BF:PR creators) uses Mumble as their integrated VOIP solution. Also Teamspeak provides full SDK for integrated solutions (link below).
Consider hiring external talents for development. ACRE, TFAR, you know...
http://sales.teamspeakusa.com/licensing.php?page=choose
http://www.teamspeak.com/teamspeak3sdk
http://wiki.mumble.info/wiki/Main_Page