Cortelco network adaptor - Manual (Page 5)

Optional Connection: If you want to use the telephone for voice
communications while the modem is not in use, connect the included
coiled handset cord to the telephone handset and the DPA jack labeled
HANDSET.
If you have connected the telephone handset to the DPA lift the
telephone handset and listen for a dial tone. If you do not hear the dial
tone, check your connections to ensure they match those shown in the
diagram below.
Return the handset to the cradle until you are ready to go on-line using
your modem and the DPA.