AT&T 2455 - Manual (Page 13)

TELEPHONE OPERATION
11
Handset Icons
Icon Description
Line in Use
On steadily with no number next to it when a parallel set is
in use.
On steadily with one or more numbers next to it to show
which extensions are using the line. For example,
12
indicates that Handset 1 and Handset 2 are on outside calls.
Intercom
On steadily with the extension numbers currently on an
intercom call. For example,
12 indicates that Handset 1 and
Handset 2 are currently on an intercom call.
Battery
Cycles (low, medium, and high) when Handset battery is charging.
Flashes when battery is low.
M M Mute
On steadily when the handset microphone is muted.
H H Hold
On steadily when the line is on hold.
R R Handset Registration
On steadily when a handset is either not registered, or is
searching for a base unit.