Are people under the assumption that Nick not returning Dana's calls or texts is a recent discovery? The way I read it is that Diaz has never bothered to answer the phone to Dana or ever text him back. These are things he's probably never done even since his early UFC days.
I remember a vlog with Dana a while back where he said that "Diaz is different, he's not like the other guys, I walked past Diaz down the corridor before and he just blanked and ignored me, we gotta work with him differently". Diaz has always been like this, and Dana probably settles all of the media/contract arrangements directly with Diaz' management team, rather than Nick himself.