There used to be a device that could be obtained from AT&T, lo these many years ago, that would isolate non standard telephone equipment from the PSTN and still provide ringing and talk current to the non standard equipment. Can anyone tell me whether such devices still exist and what they are called?
I vaguely recall hearing of such a device. In California we were spared that because the CPUC got out ahead of the FCC by establishing a certified premise equipment program. Any equipment so certified could be connected directly to the network provided it was "registered" with the LEC.
The _most_common_ such beast was a "DAA", required, pre-carterfone, for connecting a customer-owned _modem_ to the phone line.
Such devices have, for all practical purposes, disappeared from the marketplace after the FCC mandated a 'standard' isolation interface for CPE, with 'type acceptance' methodology for approving manufacturers.
Type-accepted line isolation modules are available from practically any significant component manufacturer for the telecom industry.
See, for example:
For _lots_ more, google for "telephone line interface IC".
For dialup modems, there was a device called a DAA (Data Access Arrangement). There was a manual dial/answer version, and an auto-answer version. No longer required since inception of Part 68 registration, and RJ jack types.
Protective Connecting Arrangement or Voice Connecting Arrangement. Popular varieties that went directly from a POTS line to the answering equipment included Universal Service Ordering Codes RDL RDM RDMZR RDY.
Go look up BSP's under those codes. 463-340-101 covers the RDMZR,
463-340-110 and 463-340-111 cover the RDL and RDM There is an index up at
There may still be a pile lurking in a warehouse (Sandman?) but since Carterfone (1968) and Type Acceptance nobody needs them.
It would have been nice if they made Couplers that transparently passed the line and ring voltages through, as if they weren't there. I suspect the hidden strategy was to make CPE connections as difficult and expensive as possible, so customers would give up and lease Telco equipment for that nice recurring monthly charge.
The Couplers were a pain to get from the Telco because you had to find a CSR who knew how to order them, and certain models were in short supply toward the end. And expensive to lease for what they were. But they eliminated the threat of immediate disconnection for 'attaching foreign equipment' to Ma Bell's lines.
And while they were a pain for POTS voice work, they actually made building and running a Dial-A-Joke (Guilty...) or a movie theatre "Today's Shows and Showtimes" line easier. The coupler gave you dry closures for ring voltage and line supervision status, you gave the coupler dry closures for off-hook and on-hook commands, fed it your outgoing program audio at the right levels, and it did the rest.
You only needed a tape player or two as program source with a half-watt or so speaker level output, a power supply, relays, pilot lamps, switches and a call counter to build the machine.