Return the latest call cause.
Usage example:
# place a call. If it does not return ANSWERED try again if the
# cause is BUSY
state = channel.call('sip:3301@127.0.0.1:5060;user=phone')
if state != channel.State.ANSWERED:
if channel.cause() == channel.Cause.BUSY:
# call is busy, wait a minute then try again
time.sleep(60)
state = channel.call('sip:3301@127.0.0.1:5060;user=phone')