I remember when I was a corporate lawyer, it was common practice to search for emails that just said "call me" — an indication that there was something that they didn't want to put in writing.

This wasn't a smoking gun, but it could help prove you were on the right track. Also, if you depose someone and ask them about a specific phone conversation they had, they're probably less likely to lie than if you just generally ask them if they ever talked about XYZ.