Telephone Interface

[pellmell]

I need to interface a Bellsouth BLS-420 multi-line speakerphone to a console for a project. The original idea was to use the headset jack to grab the caller and feed the mix-minus audio. I've done this many times before. However, this telephone is too smart. If I press the headset button with no headset connected, it generates an error. If I plus a 2.5mm stereo cable into the headset jack, it generates the same error. Someone, the phone is able to detect that a real headset is not connected.

The phone may be sensing that there is no electret mic element, which the headset uses. Does anyone know what I could use to fool the phone into thinking a headset is connected?

I know that there are many ways to build or buy a telephone interface, including a Telos digital hybrid, but I need to nail this one since the phone is part of an existing telephone system.

 

[ncradioeng]

Try measuring the sleeve-to-tip and sleeve-to-ring resistances on the headset plug and use those values for terminating resistances on the telephone headset jack. The telephone headset jack may want to see a DC load before it is enabled.

 

[Bengalsfan]

I have had lots of success using PBX systems with these jokers:

http://jkaudio.com/that-2.htm

 

[RolfTaylor]

Try measuring the sleeve-to-tip and sleeve-to-ring resistances on the headset plug and use those values for terminating resistances on the telephone headset jack. The telephone headset jack may want to see a DC load before it is enabled.

I concur with your assessment.

 

[pellmell]

The DC resistance of a transformer did the trick. The phone has a loud hum, yet it's transformer-isolated. Maybe the caps in the wall wart have given up the ghost.