

Unfortunately, you still need a level of trust with Proton. Even aside from trusting that they will not bend to pressure to terminate your service, you’re also trusting them with your network of contacts, because metadata (including the sender, recipient, and subject line) are not end-to-end encrypted in Proton.
I was about to say this.
If they can’t give me a callback number that is publicly listed on their web site, then they’re most likely a scammer.
With Google, however, this is a scarier proposition than with most companies. If someone from my phone company, or my bank, or my insurance company called me, I could very easily call the actual company and talk to a human to confirm. I have no idea how I could ever talk to a human at Google. I’m not sure they even have a public phone line.