Thanks to my job, I've encountered the same situation several times in the last few weeks: having to write down info (like names and addresses) that people give me over the phone. And guess what happened? I kept getting the letters/numbers mixed up, so that it took several tries for me to get it right.
The first time this happened I just thought "Oh darn, that didn't go well."
The second time I thought "Ok, it didn't go well that time either..."
The third time I was writing down the address of my piano teacher, and this time I thought "So, this is a recurring problem. So, what am I going to do about it, since it frustrates not only me but also the person on the other end of the phone?"
I quickly decided that the next time I found myself having to write things down like that, I would say "Please spell things out slowly, because otherwise I'll get it all mixed up."
And guess what? I discovered today that my solution works! I was very pleased to find that out. The thing I like about it is that it's so simple.
I suspect that my mixing the letters/numbers up is probably due to my dyslexia. It isn't nearly as bad for me as it is for some people, but it pops up from time to time.