First Google hit for Avaya 3920 wireless is the installation and administration manual

The PARTNER system views your Avaya 3920 Wireless Telephone as if it is an
MLS-12D telephone. However, there are only six programmable buttons on your
Avaya 3920 telephone, rather than the 12 buttons that are available on the
MLS-12D telephone.
The Avaya 3920 Wireless Telephone provides eight buttons. However, the first
two buttons (from the left) on the top row are used as intercom buttons.
In order for the Avaya 3920 Wireless Telephone to operate correctly, perform the
following administration:
1 Remove all line appearances from the extension to which the Avaya 3920
telephone is connected.
2 Assign up to six lines or pools to that extension.