I've made 4 phone calls this year: to a buddy, to my wife, to an old friend, and to my fax/voicemail provider (JConnect) which decided to terminate voicemail service on the phone number I've used since the 1990s. (So I may be part of the problem I am describing — that number didn't ring any device, it only accepted voice messages and faxes, which it forwarded to me via email.)
Of those 4 phone 2021 phone calls:
1.) My buddy no longer has voicemail on his phone, and a recording just told me he was not available and to try again later. (Head kinda exploded.)
2.) My wife also no longer has voicemail! How tf did that happen? No idea but it was 2 months after the above call so I was less surprised.
3.) My old friend is old-school (hence the phone call), and still does have voicemail — but it was full. I assume because he never checks it.
4.) JConnect support's touch-tone B.S. support system did disconnect me a couple of times, requiring that I call them back... but it did eventually connect me to a support rep who helped me cancel my account. But: only because they are the kind of asshole company that engages in the common-but-should-be-illegal practice of letting you sign up on the web but only cancel by phone, as a method of retaining "customers" who no longer want their product.
So n=1 sample, might not be typical, etc. but I suspect the phone is getting less universal and less reliable over time.