I seriously doubt that BA has a phone number through which any retired staff member can be contacted. If they do, I doubt even more that they would make it public, or that any responsible FT member who knows it (via BA employment or any other source) would post it in a public forum, or even pass it on via a private message, without a much better explanation of why you want it.

Added in edit: If I were a retired BA staff member, and were contacted through such a phone number (if it exists, which I doubt) by someone I didn't know, I would be both upset and suspicious.