The GPO circuit diagram is
N1291 Hopefully that will help - be careful as it usually involves mains wiring. The Klaxon is the 'Hooter No 2. The Hooter 2A or 2C are mains operated whilst the other Hooters No 2B and No 2D are DC hooters. The ringing current on a PSTN is still the same as it has been since many years ago so should still work. Hope you're some distance from your neighbours ?