How to boost audio volume coming from a telephone

Question

So I have this circuitry

Purpose is to connect the soundcard with a telephone to also play computer generated samples over phone.

It works with one drawback: The Headset's speaker is low volume. How to boost the Headset's speaker's volume? When picking up the telephone handset the volume is high enough but using the Headset's speaker it isn't.

• 4p4c goes to a telephone plug
• LSteil means the part of the telephone that emits audio data, ie the speaker
• Mikroteil means the part of the telephone that receives what is being spoken ie the microphone
• KopfhörerEingang goes to a Socket to put a Headset's headphones plug in
• the connector at the bottommost goes into the headphones output at a computer's soundcard
• Übertrager 1:6 is an audio transformer/transducer
• Masse is ground
• Links is left
• Rechts is right
• Signal is signal

Answer 1

Pity this question goes unanswered for such a long time. :-)

Obviously, the easiest way to bust the volume is to buy a cheap, specialty phone amplifier, such as those used by people with weaker hearing.

Random example of such a device: http://www.radioshack.com/product/index.jsp?productId=2467974