AT&T MERLIN LEGEND Release 2.0 Analog Direct-Line Console Operator's Manual page 55

Features
Press a programmed Feature button and dial a feature
code.
Many features have a one-, two-, or three-digit code. If
you have no more available line buttons for
programming a feature, you can access additional
features by dialing their feature codes. All feature
codes are listed in Appendix A.
NOTE:
Using a Feature button or a button with a specific feature
programmed on it are the recommended access
methods. You can use # instead of a Feature button only
in the following circumstances:
When you have lifted the handset (or used
Speakerphone or Speaker or a headset) on an
ICOM or SA button and hear a dial tone. You
must not be on an outside line — that is, you must
be listening to the system's dial tone, not the
telephone company's dial tone.
When you are connected to another party on an
inside or outside call.
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Programming a Feature on a Button