Ground-Start Button - AT&T MERLIN LEGEND System Reference Manual

Hardware
Figure 5-29 Ground-Start Button
Ring Generator
The 129B Frequency Generator (generator) must be added to the 391A or
A1 power supply module when a 012 basic telephone module or a 008 OPT
module is installed in the carrier. It provides a 105-VAC, 30-Hz ringing current
used by the ringers on the single-line telephones connected to these modules.
5-54 Accessories