Can you test it without push to talk first to get that working? Once you got that working, you can integrate push to talk.
Are you running the packaged game over internet with steam? If not, do test that.
You might need the later one here in your .ini file
[Voice]
bEnabled=true
Can someone confirm that steam voip works in 4.15? It is already implemented in my project and has worked fine untill the latest release, it no longer functions.