If they do clamp down on this robocalls can they also clamp down on junk and scam emails?
Part of the problem is that emails work differently from phone, as a meta-service. Email, having a known but outdated distributed use standard, is harder to change than one with a fixed infrastructure and a mutable backend. From your perspective, it doesn't matter how a call gets routed, or by what hardware. You send keypresses via tones to a digital encoder, and that's it from your side.
With email, there's a specific packet that goes to a specific server on a specific port number containing information laid out in a specific way.
We CAN enforce email signing, but unlike phone numbers, email addresses aren't regulated in that way, and requiring all email providers to issue certificates for email addresses would require a huge overhaul to the DNS system and IP address negotiation, since then, a domain name would have to register for a certificate.
This would absolutely allow providers to get their registration revoked for originating spam, but it would cost billions and require international agreement. It would also produce existential threats to the internet itself, as it would allow third parties to use government power to unilaterally revoke certificates.
While this is possible with the phone issue, it won't, because there can be an automatic, explicit remedy for those who violate that does not restrict their access to good faith service use:nothing past the switch is anything but analog in most situations; first there's nothing to hack or virus.
For telephony it is a bit stickier because of viruses, exploits, and backdoors getting access to the certificate store.
One solution would be to issue CAC readers for analog/landline phones to house a card that would sign, and also have a rate limiting feature in it's signing process, and just require a similar device (SD/USB/etc.; It can be many formats because the device is responsible for secure signing).
As FCC dictator, I would see to it that everyone is able (but not required) to have a user-generated public key, posted as a public on a "public ledger" block chain run by an open technology managed by the FCC, where we pay "mining fees" in a publicly managed digital currency (with which may legally be used to purchase alternative currencies at a taxed rate) to post transactions to the ledger. Everything from legal documents to contracts could exist if we had a public ledger, on which the government had some control to condemn a transaction in the official, legal fork, or was required to sign certain transactions for network acceptance with some level of certificate. You could load your mining fees at the post office, with money.
Maybe if we authorized postal banking at a level above the FCC, those fees could come to represent actual dollars, entered by serial number and then secured physically, to have the transaction re-signed with digital currency at the mint systems, if the currency is to be spent, or recirculated. All money robbed from a postal bank would be marked bills, so robbing one would be pointless.
There's a lot a public ledger could accomplish. Though this would make the FCC one of the most powerful parts of government