What is Congestion?

When i call a number, or a sip address, I get CONGESTION Why ?

When you call a number, and there is a channel Busy, it means that the SIP address you are calling the switch is giving a ALL CIRCUITS ARE BUSY NOW signal.

In a communications switch, a state or condition that occurs when more subscribers attempt simultaneously to access the switch than it is able to handle, even if unsaturated.