The third parameter, “m”, starts the music on hold instead of the ringback tone while dialing, you can also specify a music on hold class just after the “m” parameter.
-- Started music on hold, class 'default', on channel 'Zap/67-1'
-- Zap/49-1 is proceeding passing it to Zap/67-1
-- Zap/49-1 is making progress passing it to Zap/67-1
-- Stopped music on hold on Zap/67-1