Some ways for callers to authenticate themselves:
- Route all outgoing calls through a single official phone number
- Introduce themselves by disclosing a shared secret (sort of like QubesOS' anti-evil maidQubesOS' anti-evil maid)
- Distribute a dedicated smartphone app that validates their numbers
- Liaise with apps like TrueCaller to validate registered trademarks with the equivalent of Extended Validation Certificates
This list will be updated as more ideas come to mind.