The call has been put on hold by the local user, e.g. using the Hold interface.
This flag SHOULD only be set if there is at least one Content, and all Contents are locally held. Otherwise, in transient situations where some but not all contents are on hold, UIs would falsely indicate that the call as a whole is on hold, which could lead to the user saying something they'll regret, while under the impression that the other contacts can't hear them! This flag exists as a simplified proxy for HoldStateChanged, to reduce the number of signals that need to be listened to by a simple UI.