Calling may work if you get a CSR who is willing to track down the information. I used to fly UA exclusively before switching to DL about a decade ago. At some point prior to switching back to DL, I had a SM number but had no idea how to find it when I came back to DL. A call to CS with a CSR who was willing to do some searching found my old account and was able to provide me some information off of it, despite the fact the miles had expired (days pre never expire miles).