Right now, MWO doesn't have a proper built in VOIP system for team communication, you can use the C3 voice coms addin that PGI pushed but a lot of players use the traditional Teamspeak, Ventrilo or Mumble voice servers.
The available teamspeak servers are broken down into Open Teamspeaks, so factions, merc units and lone wolves can use them. And Open House Teamspeaks which are open to players more orientated to a specific faction, although generally most Houses often have guest players from other factions or merc units running off them.
Edit: Unless stated otherwise, these are Teamspeak Voice Chat Servers
Open Teamspeak Servers:
Comstar North America: Address: na1.mech-connect.net Password: WordofBlake
Comstar Europe: Address: 109.228.29.191 Password: Laserswor (No its not a mistype here)
NGNG Outreach Server: Address: voip01.n1585.hypernia.net:9992 Password: mechwarrior
Open House Teamspeak Servers:
House Kurita:
House Kurita Hub Server: Address: housekurita.teamspeak3.com:6619 Password: Yakuza
3rd Takata Lancers: Address: Black-Foxes.enjinvoice.com Password: N/A
Ronin Legion Server: Address: ts60.gameservers.com:9306 Password: R0n1n
Night Stalkers Server: Address: ts.nstalkers.com Password: Yakuza
Arkab Legions Server: Address: ts3.arkablegions.com Password: Arkab
9th Sword of the Dragon Server: Address: ts3.9thsword.org:9988 Password: N/A
HKunits.com: Address: ts.hkunits.com:9987 Password: n/a
Silent Nine: Address: 63.240.145.69:9012 Password: n/a
House Davion:
House Davion Hub Server: Address: 50.22.145.232:7345 Password: N/A
Free Rasalhague Republic:
Uses the C3 Chat Program, or can be found on Comstar NA
House Liao:
House Liao Actual Server (Mumble): Address: mumble.houseliaoactual.com Port: 64738 Password: mellon
St.Ives/House Liao Server: Address: HLVoice.com Password: N/A
House Liao Voice Server: Address: houseliaovoice.com Password: N/A
House Marik:
House Marik Server: Address: housemarik.enjinvoice.com Password: N/A
House Steiner:
LCAF Communications Hub: Address: ts1.housesteiner.com Password: N/A
Mercenary 'Faction' Voice Servers:
Darkstar Alliance: Address: atl01.bargainvoice.com:7465 Password: N/A
With regard to the Clan Factions, I haven't found any links to separate voice servers, that are in public listings, however if any unit or faction group wants me to add to this list feel free to drop me a line with the details.
After posting this thread up in the New Players Section, it was pointed out by another helpful member that there is a list of some Voice Servers in a completely different section of the Forums. I however will continue for as long as I can post and edit here because as far as I am concerned the section it was placed in doesn't leap out that it contains the lists.
The other thread is: Unofficial and Community Servers
The required voice chat applications can be downloaded from the following links:
Teamspeak
Mumble
C3
Vent
26/12/2013 Edit: Updated some of the links
Edited by ManaValkyrie, 26 December 2013 - 12:18 AM.