For the 9600 series and J100 Series on H.323, the Contacts button is a personal contacts directory where you can add your own, similar to your mobile phone.
You can add something like 250 entries, depending on the firmware (Avaya kept increasing for a while). To add a contact, press the CONTACTS button and then the NEW soft-key (as labeled on the display).
Welcome to the year 2000 and texting on a flip-phone using the letters on the keypad.

What you may want is the CM station directory that lets a station see every name programmed as a station. You'll want to program buttons for the features Directory, Next, and Make-Call. I'm assuming your stations are programmed "Last Name, First Name".

1. Press the Directory key.
2. Enter the user’s last name by using the number pad.
3. Press the Next key to scroll through possible matches.
4. When the correct user is displayed, press the Make Call key to dial.

For example, if you want to dial "Smith, John" then you press Directory and start entering the numbers that correspond with the name S=7, M=6, I=4, T=8, H=4. You can then use Next to scroll through all the Smiths to find John.

Very helpful answer. Star ⭐️ for it. Thank you 😊.