First of all, I am going through designing all of this because I did not find a ready made circuitry for my purpose which is mentioned later. If you know a circuitry that is well tested and does what I want, by all means share it.
So I have this circuit:
It is a computer-soundcard <-> telephone interface with Headphones to monitor.
Power supply is to be a symmetric 9V Power supply. PC Speaker(top-left) is the headphone output jack. The hum is in the headphones(bottom-right).
When connection to PC speaker is plugged in and Telephone connection(bottom-left) is plugged in, I hear humming in the headphones. When only one is plugged in I hear it without distortion. Sound files showing humming.
(Solved) When playing audio, it is almost not audible on the telephone side, ie, the person I am playing it to cannot hear it.
Please share ideas on how to solve the issues.
If you criticise my circuitry, add something to act as a replacement, too. Just saying that it won't work because of this and that is not as helpful as saying: "It won't work because of X, which is why I recommend doing Y". I try to update this question with the current circuit so you don't have to scroll through all replies to get the current status.
PS: Circuitry for telephone connection taken from a recording interface from Tekniikan Maailma magazine. I chose it to be able to use the circuit in parallel with my current phone.