As much as I hate to burst your bubble on this, I think most people have some amount of latent psychic ability. In the Pagan community, having a psychic gift doesn't really make you a special snowflake, because there are so many other people who have the same abilities in varying degrees. The problem we run into is that we're typically told that it's hogwash while we're growing up -- stuff we think we "see" is chalked up to a vivid imagination or too much television -- and by the time we reach adulthood, we've sort of suppressed all that natural intuition that we have. In short, many people are raised to "disbelieve" in psychic ability, so by the time they're grownups, they doubt that it's even possible.
Also, keep in mind that the events you describe aren't unusual. If your grandmother was elderly and ill, it's not unreasonable to assume that she would eventually pass away. Likewise, with the phone calls -- while it *could* be a psychic connection, it could just be that the person who called you is someone who calls regularly enough that it's not weird for them to have called at that particular time.
It's not uncommon to have intuitive flashes like you've described, and there's nothing supernatural or ooky-spooky about it. Being psychic has nothing to do with "special powers." It's more about being in tune with your own intuition and subconscious. While some people are certainly more psychically gifted than others, there's no reason you can't work on developing your own talents and abilities: Developing Your Psychic Gifts
As to your other question, about whether this somehow makes you "more powerful" as a witch, the very notion of "more powerful" is pretty subjective. I wouldn't worry about being "powerful" so much as I would suggest you focus on developing your skills so they'll be of some benefit to you.
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