Dave's advice is certainly sound but there's no knowing if the wires going to the dial are the standard colours or not. What I'd do is disconnect the dial altogether and then move the red wire in the line cord to T4. If the volume is then normal the trouble is with the dial or its connexions.
The receiver itself
could be at fault — it's unlikely but if you don't test it you can bet it will be
. To check it remove the brown and blue handset wires from the telephone and connect them to an MP3 player or some such and see if you can hear music.