on the "change system features", one of the pages near the end will have "24XX/64XX/84XX On Hook Dialing?" stroke this to a "y", then you will be able to press the speaker button, to make calls, without picking up the handset.
There is also another thing called "hotline dialing" for 2500 style sets, for that you put the # you want dialed into a personal/group or system speed dial, and specify the entry on the station form. When you pikcup the handset, the switch dials the # you specified in the speedial. This would be used for an elevator phone, or a cab stand for example.
Mitch
AVAYA Certified Specialist