AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 211

Applications
If a Key system is being converted to a Hybrid/PBX system for limited
Centrex service, the conversion from loop-start lines to ground-start lines
can cause a long delay before the system can be installed.
Extension numbers should reflect the last four digits of the Centrex
telephone line number. A brief ring delay occurs when calling a Centrex
or PBX host extension number because the call is being processed
through two systems. No delay occurs when making a system intercom
call.
Centrex Operation 4-55