https://downloads.avaya.com/elmodoc.../upam_universal_paging_access_module.pdf

Set MODE switches S3 and S4 for Pre-announce or Confirmation tone (as desired)
Set MODE switch S5 in the OFF position
Set AUX switches S3, S4 and S5 to ON position


Some phone systems (in particular, Centrex-type systems) may produce openswitch-intervals
(OSIs) when the UPAM first answers the line. OSIs are short
breaks in loop current resulting when the central office switches equipment on and
off the line. The UPAM may misinterpret these OSIs as disconnect signals. It may
be necessary to set AUX switch bank switches S4 and S5 in their OFF positions
(AUX switches are located on top edge of unit - see Figure 5 on page 16) if the
UPAM exhibits any of the following symptoms during station mode operation.

UPAM disconnects from the line immediately after answering