Trine - Almost the same thing happened to my bff last year on a school trip!
So, almost a couple of years ago we went to the beach with a friend, where we were meeting our guy friends. One of them came to us and we thought he wanted to show us were his umbreallla was, so we followed him. He stops under an umbrealla and we're like "that's his umbrealla, we're leaving our stuff here". At sometime a phone, identical to his rings and my friend tells him "(insert name- which is actually an abbrevation) your phone's ringing" and he's like "Nah, leave it, it's OK" and he goes to swim. After a minute or so, a random guy, we've never seen in our lives, comes under the umbrealla, too and says "is (insert the full name of our friend, which no one ever says) umberalla?" and we say "do you mean (insert the name we call him)?" and the guy just shrugs and takes the phone we thought it was our friend's to talk. And on the top of it, when we said we made a mistake and we are sorry the guy is like "oh, it's not a problem, you can stay here with me". Then we went to our friend, who didn't understand not a thing and he started laughing like an idiot. Now that I'm thinking it's actually quite funny, but at that point it was downright embarassing!