It's not magical, but "could I get that in writing?" can be surprisingly powerful.
You do have to be persistent & leverage documented communication as much as possible to get results.
Communicate over email if at all possible, & if you have a phone call or an in-person meeting about it, take notes & then send them in an email to all persons involved. Every single little convo you have about it, email the person & say "thanks for the talk, here's the content of our conversation."