[Zak]

Sure, but with phone numbers that can't be spoofed, telcos can terminate service, and filtering technologies can block calls. Spam gets expensive if you have to buy new service every five calls.

[singpolyma3]

It does. But the spammers still do it. Because eventually they hit one person who gives them a thousand dollars or whatever and it pays off.

[Zak]

Preventing spoofing doesn't have to make spam cost-prohibitive for every spammer to greatly reduce the volume, and it does not interfere with ordinary people obtaining phone service anonymously.