As someone who has worked in telecom for a long time, I assure you most of us carriers / service providers (Except maybe Big Red and Big Blue) desperately want to implement signed SIP invites and kill robocall for our customers. We also want to universally deploy G.722 HD Voice without unnecessary transcoding. The telephone industry and the Big Color Telecoms (Red /Verizon, Blue / AT&T, Green / CenturyLink, Yellow / Sprint, Pink / T-Mobile) moves at a snail's pace. The security model of domestic VoIP right now is largely IP whitelisting or private IP circuits, no encryption allowed unless it's an entirely on-net call where you deal with that yourself and send it to the telecom unencrypted. We'd like to fix that too, but again, big telecoms aren't interested as best we can tell (they don't communicate their strategy reliably so we have to infer from their actions). Encryption of in flight calls, even with wiretap laws and legalized domestic surveillance, is important in my opinion.

Of all of them, I think T-mobile is the most willing to actually improve the industry for everyone's benefit.

When we receive a SIP invite (incoming call) from one of our upstream carriers, the amount of metadata information they're able to provide us is often very limited (largely due to interop issues in my opinion). If you think about IP spoofing, BGP, and the state of egress filtering on the internet, it's a very similar situation.

Since this is related to my work, obviously all of the above is just my personal opinion.