An old GPO engineers maintenance trick for fixing the problem of the earpiece being quite on an early Bakelite 332 telephone is as follows:
1) Unscrew the bakelite earpiece (anticlockwise)
2) Slide the metal diaphragm (thin metal disk) sideways off the earpiece magnets (make a note of which way round it is)
3) Make sure the two screws either side of the magnet are tight.
4) Spray the metal diaphragm (both sides) with a small amount of WD40 and wipe off with a piece of paper kitchen towel.
5) Replace the metal diaphragm (the opposite side up to the way it was removed) by sliding it sideways back onto the magnetic coils.
6) Screw the bakelite earpiece back on.
Then try the telephone again and see if this improves the sound (works about 50% of the time).