In other words, 'we don't want to put anything in writing'. I have often had this response when asking awkward questions from lots of companies and organisation.
This is how my DH deals with this problem at work:
take notes during the call, summarise the salient points in an email (referring to the date and time of the call and the person you spoke to). Send it to the contact you are dealing with, informing them this is your understanding of the other side's position. Then ask them to correct anything you got wrong.
Thereby you get written evidence, in one of two ways:
either they don't respond, in which case, as per your statement in the email, they have accepted your summary.
Or they dispute your summary by responding to your email thereby putting their position in writing. If they call you to correct whatever errors they think you made, you repeat the process.