Originally posted by Tarry

the conceptual difference
The phone number you dial is the number which the other end must possess.

the use of a listener.

A telecom exchange must be ready to not only wait/standby/listen for your calls/requests but for everyone else. So after you have been connected, the exchange reliquishes the connection and waits fro others to call in...

Hope this explains a littel bit.

Classic explanation. Thanks a ton tarry.