If you watch some of the ROK streams, you will see that the voip works just fine. It doesn't work at all on DayZRP server. Some people say changing the push to talk key from v to something else breaks it for that person, but I cannot confirm this personally.
I think this is a server side issue - setting, hardware, version, network - whatever... It could even be the world running on an old version. Server wipe (which was suggested by the devs after applying the newest update) could remedy it, but I don't know for sure.