If you are calling a virtual meeting room from for example an unregistered Polycom system or other endpoint that can not call URI addresses then you need to use the meeting room alias and an IP address.
Call Meeting ID@188.8.131.52 or 184.108.40.206##Meeting ID
Meeting IDs are unique for each virtual meeting room. You should have received these together with the URI addresses for each room.
It is usually 6 or 12 digits long
When you are asked to enter PIN code you have to use #PIN# to join.