Comment Should rather be based on SIM cards or numbers (Score 1) 165
People will happily sell their old phones for as low as $10. You can probably buy old, usable phones by weight.
Any system based on tracing guilty people by phone purchaser in any scale and with automated means, would identify countless innocent people.
In my country there's no registration for buying phones, but strict ID requirements for SIM cards. This means spam calls are mostly from visibly foreign numbers. Numbers must be kept up to date with a subscription, numbers won't be thrown away.