I have been asked to look into the feasibility of creating an
> interface to a standard telephone (via an RJ-11 jack) which meets the
> following requirements:
>
> 1. Provides sufficient power for standard line-powered
> telephone. (+5 VDC and +12VDC are available - step-up via
> DC-DC converter?).
>
> 2. Simulates a standard "ring" signal. (115/6.3v transformer
> connected "backwards"?).
>
> 3. Detection of "off-hook" condition (TTL level output).
>
> 4. Performs 2-wire to 4-wire conversion (standard ICs available?). >
> 5. Decode DTMF signals (plenty of ICs available - any favorites?). >
> Note that none of this needs to meet any agency approvals since
> there will be NO connection to the Telco.
>
> Any hints, suggestions, pointers to reference materials, app notes,
> schematics, etc. would be VERY appreciated! So far, I've managed to
> bury myself in various texts, but I am clearly out of my element here
> and I have the deep suspicion that I'm trying to re-invent the wheel -
> and that there may be others out there for whom this would be
> considered trivial.
>
> Can any of you telephony wizards help? I should mention that IF
> this project actually got to the commercial development stage (and
> that's a big IF), I could actually afford to PAY someone who could
> contribute their expertise. In the meantime, I'm trying to determine
> if this is actually feasible at a reasonable cost OR if the marketing
> dreamers should be sent back to their cave :-).
>
> All input and/or comments welcomed and appreciated!
>
> Regards,
>
>
> Bill
formatting link