AT&T 874 User Manual page 11

INSTALLATION
7 Plug the power adapter into a standard electrical outlet.
Convenience Port: If you want to connect another device (such as
an answering machine or fax machine) to the wall jack, you can use
the jack on the phone labeled AUX. This convenience port will
always use Line 2; a call picked up on Line 2 at another extension
may interrupt a fax, modem, or message transmission.
Convenience Port
9