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 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.

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 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.

Some ways for callers to authenticate themselves:

• Introduce themselves by disclosing a shared secret (sort of like QubesOS' 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.

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 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.