Voice over Internet Protocol (VoIP)
Voice chat in APB allows players to communicate with one another, even cross faction. You can hear people and talk with them when they are nearby. People in groups can hear each other over any distance.
When a player joins a group, if a mission is already being played in that group, the player will be unable to communicate via voice chat until that mission is over. In the mean time, the player can accept a mission of their own, effectively having the group run more than one mission at a time. The reason for the limitation in voice chat between alternate missioned groups is presumably to keep chatter about a mission constrained to those who are participating in it.
The default voice chat mode is voice activation. There appears to be no voice activation level setting currently however.
Push to talk mode is available but many players have had difficulty with VOIP after setting push to talk, such as not being able to communicate with other players at all.