Directly from the system manual...
Hot Line
Description: Individual telephones can be programmed through CDE as hot lines. When a hot line telephone goes off-hook, the system automatically dials a pre-programmed number (internal or external). The number is specified as the call forwarding destination for that extension.
The following conditions apply to this feature:
• The extension must be programmed as a Manual Line.
• Manual Line and Automatic Callback are mutually exclusive.
• The extension must have access to Abbreviated Dial, and External Call Forwarding (if it is to dial an external number).
• The type of forwarding programmed has no effect on the feature.
• If the extension is called, then the forwarding feature still works as usual.
• The hot line call is a reroute to the forward number; see Call Rerouting.
• Valid hot line destinations include all valid call forwarding destinations.
• A DTMF receiver circuit is needed for the hot line call regardless of the type of the destination.
Programming:
Enable COS Option 228 (Manual Line) for the extension.
The attendant must program a “Call Forward - Always” destination number for that extension; see Attendant Call Forward Setup And Cancel.
If programming the destination number from the extension then enable one of the call forwarding COS options in the extension’s class of service.
Operation:
Lift handset; the extension is rerouted to the forwarding destination automatically.
Although for some reason it seems to me this rings to the Attendant extension for this tenant from Form 19, not the CFA destination as stated in the manual, but it has been a while since I have set this up.