I want to talk to four friends in private while gaming. I know of Skype, but that is not ideal as "Push to Talk" is limited (I can't set it to my mouse 5 button).
I've heard of Teamspeak, but I need to pay for a server?
The Teamspeak software is free of charge. You can setup your own server and use the clients as much as you want. Just download them from here.
But a Teamspeak server requires bandwidth and some other resources, so there are different service provider that charge you for a Teamspeak server.
That said, here are your options:
Personally I own a virtual server and have setup my own Teamspeak server. It is not that hard to do.
The TeamSpeak server software is completely free, but running it may involve additional costs, including a decent connection to the Internet with an upload speed from 1-3 Mbps. Don't worry, you can always tweak the voice quality if what you have isn't good enough. On top of that, you'll need a computer with plenty of RAM (hopefully more than 2 GB) so that you can run the server while playing.
Just remember to forward the necessary ports (9987 udp for voice, 10011 tcp for serverquery, 30033 tcp for filetransfer, and 41144 tcp for tsdns) on your router.
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