That "strong sense of familiarity" rowanjenkins mentioned is key. Maybe @averyc80's brain linked the feeling of forgetting to a past time they forgot something else, creating a false echo. It's a weird mix of memory failure and deja vu.
Actually, forgetting your number is probably just a brain fart, not deja vu. Deja vu is when you feel a STRONG sense of familiarity with a whole new situation. Like walking into a room you've never seen but feeling like you have. Forgetting a phone number is more about memory slip, like blanking on a friend's name. Your brain just hit a temporary glitch, nothing mystical. It happens to everyone all the time.