Connecting Desktop Audio Jack to Landline Phone Handset Port: Feasibility and Design

I have a small problem at work that I'm trying to work around until we can get a more technical solution implemented for our system. I'm looking at patching a  standard desktop audio jack (used for a standard headset with mic and speaker) into a landline phone. Essentially connecting the computers audio in place of the phone's handset. Visual description and possible design attached. Can anyone tell me if this is going in the right direction?

Lukas...looks like you have opened up a handset and got the colour codes of the wiring.  Don't worry about the resistors unless you need to attenuate the signal .  With a multimeter, investigate the wires and see if you have any DC voltages between the speaker and mic wires.  Computer audio ports have separate 3.5mm stereo jacks for Mic and Speaker and the grounds are usually tied together, so you don't want to short out any DC voltages in the phone.  Use capacitors (Non polarised if possible) on all legs if necessary. But if there are no DC voltages present you probably won't need the caps either.  Tie the non-earth side of each connection to both of the "live" connections on the 3.5mm plugs (ie parallel Left and Right).  If you really want to be professional, use 600 ohm 1:1 transformers on each side (mic and earpiece).Back to the resistors.  if you're using your computer's speaker jack, the signal may be too much for the phone and you may have to attenuate it.  Similarly the phone's output may be too much for a mic input (if your computer has line in / out ports, use those rather).  A 1K pot should work find for both if needed.

Hi Lukas-I agree with David except for one thing- the 600 ohm isolation transformers he mentioned are not optional- they will be required.  Otherwise, at best you'll have lots of AC hum because the ground potential of your phone system and for the electrical distribution system will not be exactly the same. Also, on some phones systems there is a lot of DC on the handset components in relation to ground.  Leaving out the transformers may make your phone appear to be "off hook".
-Rick 

Good point Rick.  if you were working to a laptop running on batteries you'd be ok, but not with an earthed desktop....Handsets usually have a simple earpiece and microphone, but they may not be electrically commoned.  Headset ports usually have a DC present for the headset electronics.  Either way, transformers are a good idea.

The telephone usually requires the microphone in the handset to have DC continuity, as the DC current powers the microphone. The 600 ohm 1:1 transformers are definitely required. If you want to parallel the left and right speaker outputs, I recommend putting 10-ohm resistors in series with left and right outputs and taking the sum from the junction of the two resistors. This permits the difference between left and right signals to see a 20-ohm load rather than a short circuit. Take a careful look at the components inside the handset you want to emulate, to see what they are and how they function. Look up "amateur radio phone patch" to learn more, as hams have been doing this for decades.

Where dialup sends only one voice or data signal down one channel, broadband divides the line. Telephone lines were never designed to carry computer data vidmate.