> The problem with SS7 is that you extend your SMTP analogy, there is no way to implement the equivalent of SPF, DKIM and DMARC for verification of incoming traffic without breaking SS7-to-SS7 links between the vast majority of installed phone switching gear out there on the PSTN.

Why not? That's how it was done for email. SPF doesn't prevent interoperability for sending domains that don't use it or recipients that don't verify it. What it does is inhibit forged messages from a domain that does use it to a recipient that does verify it.

Then the more senders who have an SPF record, the stronger a spam signal it is for a domain to not have one, and the stronger the incentive gets for senders to use it. It's already in the recipient's interest to verify it when it's used.

They could even use DNS for this exactly like email. Create a lookup zone equivalent to in-addr.arpa but for phone numbers.