When this is connected the LED flashes very faintly and the buzzer does not sound at all. We have checked all the components are OK & the circuits soldered correctly. We noticed that the circuit diagram shows a 470nF capacitor, but a 1uF has been supplied. Might this explain why it is not working?
The capacitor must be 1µF, supplied components are correct.
Check the zener diode’s there polarity and “if they are not swapped” (12V / 20V)
Finaly, be sure you are using a PSTN network.