Service responsible for making and receiving calls.
Note that most actions on calls do not return an error state. The reason for this is the current call states is already reflected through the currentCalls property, which should be reflected by the UI directly. Any error handling would only make this complex and error prone. However the service implementation is encouraged to log any reason why an action might be ignored as a warning.
Instructs the call with specified callId to play a DTMF tone. The tone will be played for a fixed amount of time before being stopped. Any other currently playing DTMF tone in the specified call is immediately stopped.