Hi @kurmasz ,
I'm fairly sure that the handset has an audio amplifier because of its' ability to adjust the volume up and down.
The power for the amplifier, which is most probably incorporated in the main circuit board would be derived from the main battery supply.
You can prove whether or not the headset volume is also amplified by using the Up/Down arrow keys during a call and listening if the volume goes up or down accordingly.
If the headset volume cannot be adjusted with the volume controls during a call then perhaps it is a fixed level as they do not wish for the audio to be distorted or the headset to be damaged (or your ears!) by having the ability to turn the volume higher
Since you say that it is really loud and if the volume can be adjusted, then perhaps the headset jack is on the same audio path from the amplifier to the main receiver. Another pointer to this is that the insertion of the headset cuts-off the main receiver.
Therefore the reason for the low level in the receiver could be either a faulty connection on the phone's receiver, a faulty receiver itself or perhaps (and I don't know if this is accurate in your case) there may be a faulty contact in the headset jack. (some headset jacks use an internal set of contacts - internal to the jack itself, that physically changeover the audio path from the receiver to the headset when a plug is inserted, whilst others use a contact to signal the audio controller to divert the audio away from the receiver and onto the headset)
Hi @kurmasz ,
The only way to conclusively prove it is to open the phone so that you have got access to the jack and test it for low resistance continuity by using an Ohmmeter.
Most likely it will be soldered onto the pcb so that it will be difficult to test. Most probably it will be an SMD type component sitting on top of the board and not a thru-hole type of component.
It may be easier to unsolder it from the board and then test it "out of circuit" to verify whether it is faulty or not. If it is faulty, as you cannot repair them, you'll have to source a replacement part. Depending on where you are try either searching online at mouser.com or au.element14.com
This is an electrical representation of a jack. Yours may not be so complex but it shows the changeover contacts that would change the direction of the audio
(click on image to enlarge for better viewing)
Audio from the amp would appear at poles 2 & 6 in my example and go out on 3 & 7 to the speaker when the headset is NOT inserted and this connection would be broken when the headset is inserted and the audio would be directed out to the headset essentially on the springs connected to 4 & 8 - Apologies if you know all this