I can't say what I would do as my friends, like myself don't vet calls and therefore do answer their 'phones. If they were not available I know they would respond to my voicemail as soon as possible, as I would theirs.
I find this modern practice of picking and choosing what calls to answer to be extremely rude.
Well this hasn't happened to me since I was a teenager and there were no social media sites back then, in the Dark Ages!
I wrote her a letter, explaining how hurt I was by her behaviour and she reacted very badly, so that was the end of the friendship. Toxic people are best got rid of and out of your life, in my experience!
If there was someone whose calls I didn't want to answer, then I wouldn't consider them a friend - part of friendship is being able to talk to each other. So the same would apply in reverse, if there was someone who didn't want to talk to me, I wouldn't Facestalk them, I would just end the friendship with as much dignity as I could and walk away.
I'd take the hint. Also, some people hate talking on the telephone, but love communicating via email - probably because they can pick and choose when they type in a few lines and run back and stir the pot, go to the lavatory, feed the dog, brush the crumbs from the table, throw a roast in the oven - go back and have a look how it's cooking, see who just won money on a quiz show, catch the item on the nightly news you have been waiting to see, shave your legs, strip your moustache off. All those things that your forget about once you are in the middle of a long phone conversation.
It isn't being rude not to answer or call back, I think it is just that people are busier these days. I certainly don't think it is because people simply don't want to speak to people, the fact that they have telephoned you and left a message, you know that they are OK
And, if they don't leave a message when they ring - some people might think the caller is one of the very, very annoying people that ring trying to sell electricity, phones, survey seekers, raffle tickets etc. and these people often ring at dinner time.
If you don't get an answer to the phone, email your friends if possible, so much less intrusive to all manner of things like mentioned above.