+ | | - +-------+------ - - --+---||||---///--+---- - -----+-------+ | | | | | R | | | | | | 24V | | | | --- | | --- | | | | +---||------------+ | | | | --- Sonalert C Sonalert --- | | C | | C | +---||--+ +--||---+ | _|_, _|_ | | / 15V 15V / | PHONE -+- Zener Zener `-+- PHONE | | | | | | | | +-------+------------------ - - - -------------------+-------+ As before, set R to give you a talking current (both phones offhook) of about 25mA. Start with 1K ohm. Leave it in if the phones work well enough; the current is not very critical. The capacitors C are audio bypass capacitors and should be about 47uF.