My guess it is a 300 series type telphone like the one here:
https://www.britishtelephones.com/t300info.htm
Made by AEP
How the circuitry is are not easy to tell, but
1) Weak sound when speaking may be the transmitter capsule (microphone element)
2) Weak sound when litening may be the receiver. Somtimes that may be solved by adjusting the distance between the electromagnets, and the magnetic membrane.
3) Weak ringing may be weak (wrong) ring signal on a more modern exchange or ATA. It may be mechanical adjustments, or a bad capacitor.
dsk