[callchannel] Don't play tone if MT call disconnects before answering

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>

No need to play a disconnect tone when a ringing call is ended by the far end. Missed this when enabling tone playing functionality (pull request #12) which was flagged out for tp-ring 1.x ==> tp-ring 2.x migration.