Both the Survival and Creative servers have the Simple Voice Chat mod installed, which you must install on your client to use.
It is worth reading through their mod page and wiki for info on how to use, however the commands you have access to are listed below:
/voicechat help- displays commands/voicechat invite PLAYER- invites player to group/voicechat join GROUP- joins a group via name/voicechat join GROUPID- joins group via ID/voicechat leave- leaves current group