If I was an advertiser do I really want to pay to have my ad (video nonetheless) shown to someone while they're actively trying to have a phone call? As a user, I'd just be pissed off at the advertiser plus im not looking at my screen when talking.
I guess technically this increases ad "views" metrics. But advertisers are just wasting their money. I hope the fact that Meta is testing changes like this causes advertisers to re-evaluate the risks of using FB advertising.