Do you ever call someone and then, before they answer, forget whom you've called?
You dial the number and as you hear the first ring at the other end, you get up from the table to absentmindedly sort the mail, putting a dangerous amount of distance between yourself and any note to self or clues as to whom you've called. Or your cat rolls over on its back, and looks absolutely adorable, so you walk over to the couch to stroke its belly and out the window you see a bird you don't recognize and you start to wonder, "Is that bird book around here somewhere? I think I left it on the shelf in the kitchen." Or you hear the buzz indicating the end of the dryer cycle and you wander down the hall and open the dryer door to feel if the clothes are damp or dry. No matter how it happens, a couple of rings go by as your mind and your body move about your house. The ringing continues softly. You enter a meditative state. Ring, ring.
You almost forget the phone is there, cradled by your head, when suddenly there is a voice in your ear, "Lo?" Just one syllable, not even a whole word. The voice may even be a bit breathless because this person ran to get the phone during all those rings, so the voice doesn't even sound like the person's normal voice. You are off guard but you rally. You answer "Hi!" real friendly-like, chipper, and with the air of familiarity. It's me, your voice says, you know me.
And now you hope. Maybe the disembodied voice will be helpful and say, "Oh hi, sis, I'm so glad you called," or "Why hello, Cathy, I, your best friend Sharon Brown, haven't seen you since you were at my house, 20 Main Street, last week." But those scenarios are unlikely. The voice will answer "Hi" and you are left hanging without a thread.
You have to hope that your sister/ best friend/favorite aunt/tennis partner is as old and forgetful as you are and will understand when you fess up. If they are in the same advanced state of mental flabbiness, you can probably count on them forgetting this incident ever happened. And if it turns out you were calling the plumber, well, he doesn't care as long as you pay the bill.