What Is Call Waiting? Call Waiting Definition | Mitel 

What is call waiting?
Call waiting is a popular phone feature that lets you take another call when you’re already on the phone. It is like having two lines on one phone.
How Call Waiting Works

Send the incoming call straight to voicemail, and continue your current call as normal
End your current call and answer the incoming call
Place your current caller on hold while you answer the incoming call
If you place your current caller on hold, that caller will remain on hold until he or she either ends the call, or you switch back to that call and end it.
With call waiting, you can alternate back and forth as many times as you want between the two calls. Each inbound caller will alternate being on hold. The calls are entirely separate and private-the two inbound callers cannot hear one another or communicate. They will not even know that you are on another call unless you tell them.
Many phone systems also let you to turn call waiting off, which is useful if you are in the middle of a long, important call and do not want to be interrupted. If call waiting is off, inbound callers will hear a busy signal or get sent to voicemail, depending on your phone settings.
Call waiting only works for two inbound calls. A third caller will hear a busy signal, get sent to voicemail or be otherwise routed based on what other phone system features you have in place.