How do you handle prospects that have unresponsive sites and say "It works on my phone, I'm looking at it now" even though, it's totally broken, failing G responsive test and also failing visual clues?

These are well researched leads, and client on the phone (cold call) answer is receptive. So I can get to dialogue stage.

But I can't start arguing with them their site is screwed...so how do I prove it to them without sounding confrontational?

The plan is not to hang up and move on to the next call, these customers are receptive, I only need a way to positively overcome the objection...