Call channel methods

The call channel methods are:

• call
• start call
• wait for outgoing call
• wait for answer machine
• ring
• answer
• hang up
• reject
• wait for idle
  • UASCallChannel
    • UASCallChannel.wait_for_idle()
• state
• call termination cause
• connect
• disconnect
• retrievable transfer
• retrieve transferred call
• wait for transferred call retrieval
• transfer to conference
• transfer cause
• call conference room
• start call to conference room
• call inbound service
• start call to inbound service
• transfer to inbound service
• create tone player
• wait for any state
  • _wait_for_multiple_events.wait_for_any_state()