VTech IS7121 Installation Manual page 15

4.
Using another wire, slide one end of the metal part
underneath the metallic plate on the doorbell, then connect
the other end to the Trans terminal of your chime/buzzer unit
of the wired doorbell.
Option 1: Ring the front doorbell of the existing wired
Adjustment
View
Right
doorbell and the IS7121 doorbell simultaneously.
Angle
View
Left
Rear Trans
Chime/buzzer of
wired doorbell
5
Transformer
Front