I think as telecom-as-a-service grows and facilitates further and further automation of this kind of thing, we may see the growth of phone call spam filtering t

I think as telecom-as-a-service grows and facilitates further and further automation of this kind of thing, we may see the growth of phone call spam filtering the same way we have with email. Of course, telemarketing is nothing new, and so far nobody's really attempted anything like that to my knowledge. What if any technologies exist for PSTN that are analogous to DKIM or SPF in email?