Please see TS 22.084, subclause 1.3.8.4.1
 4)	Join the single active call and the held MPTY together.	
This would result in an active MPTY, except if the number of remote parties exceeds the number allowed.
	A "MPTY invoke" notification shall be sent towards all remote parties.
	A Retrieve notification (according to TS 22.083) shall be sent towards the previously held remote party.