How can I connect two cordless handsets together using the 3/16 jacks?

I have a cit 310 yahoo phone and would like to connect the headset 3/16 jack to a 3/16 jack of a landline cordless phone so I can patch the yahoo call to a cordless landline.? This way someone can call me through yahoo, I can answer on the cit310 then call another friend on my home phone, patch the two together and have a 3 way conversation by using another extension in the house.

You could build a crossover cord (mic to speaker, speaker to mic), but to be honest the results will be less than spectacular. The microphone inputs on both devices are built for un-amplified input, so you'll cause distortion by connecting an amplified source (the speaker line). Still, the parts list is pretty minimal. You'd need 4 wires and 2 jacks. You can probably find that for less than $6, anywhere in town. A modified version of the same project would have some consideration for dealing with excess noise. Anyway, this amounts to something similar to what's known in Phreaking as a Cheese Box... and it's potentially illegal, depending on where you live. Before you proceed, check that out and check the terms of service with your phone company and yahoo.