I think the only thing we can say in general is:
Is this professor implicitly telling me he is no longer willing to write me a letter of recommendation?
That would not be professionally acceptable for a professor to do deliberately. If a student asks for a recommendation, the professor should (after discussing with the student if needed) either say yes or say no, and if they say yes they should actually write the letter. If they say yes, and then something happens such that they are no longer able or willing, they should inform the student promptly. "Ghosting" would not be a normal or appropriate way to do that.
Of course, real people are human. There are many possible explanations for the lack of response, which may or may not mean the professor is unwilling to write:
Your emails may not be reaching him at all (technical problems, spam filters, etc). Phone messages also may or may not work that well - since people don't use phones as much these days, they don't always keep up with updating phone numbers, setting up voice mail notifications, and so on.
He is replying, but for similar reasons his responses are not reaching you
He sees your emails as just reminders about the due date of the recommendation, for which you don't expect a response (or for which a simple response of "I'm on it, thanks for the reminder" seems unnecessary to him)
He intends to write the letter but is not very organized about it. For instance, the professor might see your email, and say to himself "Oh yes, that letter. I think I can write it tomorrow, so I'll just wait until then to reply, so that I can just say it is done." Then he doesn't get to it tomorrow and puts it off another day, and the cycle of non-response continues. Still, even fairly disorganized professors end up meeting hard deadlines most of the time.
He is genuinely hesitant about writing the letter for some reason, and knows he needs to discuss it with you, but is putting off the potentially awkward conversation. (That wouldn't be good professional behavior, but it is common human behavior.)
He is having some sort of personal difficulties that make it hard to keep up with work, or certain kinds of work. The fact that he was able to reply to someone else's email promptly is not proof that he is 100% able to meet every other commitment.
There is no way that we, as Internet strangers, can guess which of these is going on (or if it's something completely different). The only way you'll be able to find out is by talking to him. Visiting in person, or trying to reach him through a colleague, may help.
Of course, it is in your own self-interest to have a backup in case, for whatever reason, the letter doesn't get written. This could also be a way to prompt the professor indirectly: you contact Professor Y and say "Hey, Professor X agreed to write me a letter, but I tried to confirm it with him and can't get a reply. If for some reason he isn't able to do it, would you be able to write me one?" Besides lining up Y as an alternate recommender for you, this gives Y an incentive to follow up with X on your behalf: if Y can get X back on track, it'll save her the time of writing a letter herself. :-)
