I do not know the details about this for residential telephone calls (and it may depend on your service provider), but on our business telephone line, the caller has to press "8" to leave a message. If the person does press "8" and say something, then the number, whether it is or is not available, does not get registered. I know that this may not be a best solution, but it may be something that will lessen your anxiety . . .
Now, and this only works if I am home, when I get a call, I let the answering machine pick it up and if it is someone I need or want to speak with, then I answer . . . otherwise, it just goes into the answering machine. This might be coincidence, but in the last 3 years, I may get one "bad" call every 3 or 4 months . . . no one running a scam or whatever is going to waste their time redialing a number that goes into an answering machine because these folks only make money for connected calls when they can do their speel . . . also, and this comes from discussions with others, if it is a random generator dialing the number waiting to make a live connection before an operator comes on line, after a given number of tries over a given period of time, your telephone will get marked as "bad" and taken off the recall list . . . .
I also protect my home telephone number and when I need to provide a telephone number (unless for clearly legitimate reasons), I give my pager number . . . .if it is a legitimate call, then the person will leave a message. If it is a call selling me stuff or scamming me, then after a few tries, the number is deleted from the recall list . . .
I don't know if any of these strategies will help, but that's my take on it and although these calls do not cause anxiety in me, they do frustrate the *(*(( out of me because the usually come at supper time or when I have just gotten off to sleep (I usually answer these because I figure if the person wakes me up the only thing I can do is give him or her a blast of my less than ladylike vocabulary . . . . and, quite frankly, I have no regrets) . . . take care, KC