I toyed with that for a while but I kept missing important work calls. I might have a look for an app later, but I have a feeling it might not exist...

Yeah. I tend not to pick up calls that are in the "Who would be calling me from Texas?" vein. But while it's annoying to have to look at my phone when it rings, I do get calls from locations that seem plausible and they usually are legit. I'm not really willing to make myself harder to reach for legitimate and even important reasons because of the occasional junk call.

I wonder if you can get a VoIP number from a different country (where good regulation means spam is less prevalent) and use that for work calls?

I'm almost 100% sure your employer wouldn't want to make an international call every time they wanted to contact you by phone.

Work uses slack/teams/Webex. One person sends me Signal. No one has ever used telephony, except I use it to call he dial in numbers because my phone audio is better than Bluetooth / virus agent laden laptop displaying ten videos of peoples homes thru vpn.