Our tool Sococo (I work there) works audio/video P2P so it all can stay inside the firewall.
But chat is generally aggregated/federated so ends up on an encrypted server in the cloud. That can be an issue to fix; either its saved on everyone's own machine (and not your pad at home when you want it later), or its on a server and theoretically 'at risk'.
Supports Mac, iPad, iPhone, WinXp/Vista/7/8.
We run it on Linux but only in the datacenter as a 'media node', we couldn't settle on a presentation layer.
Also we've ported it to Android pucks for conference room stations, but not selling that - no deal with a vendor yet.
There's a web client, but the audio situation there is fragmented so now (sigh) you have to call in. Its OK if you're just attending a meeting but less tolerable for all-day use.
But chat is generally aggregated/federated so ends up on an encrypted server in the cloud. That can be an issue to fix; either its saved on everyone's own machine (and not your pad at home when you want it later), or its on a server and theoretically 'at risk'.