Example:
An external call coming ... I answer it by my phone; the caller want to talk with "Mr. Smith"; then I press "transfer key" on my phone .... I don't remeber the "Mr. Smith" extension, so I'm late to dial its number... after 3 seconds I listen on my phone the "line down tone" (hope it is the right name)... so I need to bring me the line and press "transfer key" again then compose the extension faster...
PS: the time you indicated is correctly set to 20 secs