After reading your post, here's my guess: perhaps she tried calling you and couldn't get you because you disconnected your previous number. Maybe she just remembered calling you and not being able to get you but when she did talk to you about it, she just remembered that you didn't pick up the phone.
As for the number not being assigned message, I get that too sometimes when I call people. Then when I call the number again later, it works. I only remember this happening with cell phones though.