That used to happen to me too, then I studied it more, and would periodically spell words in my head using it until I recognized which letters I'd forget (kilo, lima, mike, november), then studied those some more.
Total time spent memorizing it fully probably sums to less than an hour over a couple years.
So then you don't actually know the alphabet. Knowing implies you know all the letters and can recall them at will at any point in time, - that's what makes it useful. Of course if your forget some letters, that will just make the whole thing more cumbersome than just using letters.
I can name them, but perhaps not out of blue during a phone call. It's stored deep in the back, along with the position of stars and the lyrics of Disney songs.
Really? I've found it enormously helpful when communicating alphanumeric strings (like, say, license keys) over the phone with support people in countries where English is not the primary language.
I also tend to forget letters, leading to something like "bravo uniform xylophone"