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Looking for Suggestions for an Intercom
(lemmy.world)
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If you can find landline phones with that feature set, just get them. You can just wire them together and put 20 volts DC in the line. Of course you're not going to want to do that over your single ethernet run but if you had multiple...
Telephone services really simple, 18 to 24 volts on hook should pull down to about eight or nine volts when you open the line. If you pulse a 90 volt signal, It will cause them to ring.