I'm mostly a buyer of Adwords, not a seller through Adsense, but it seems to me that Google's algo is designed right here... if the ad creates click, its quality will keep the ad on the page. If it doesn't, then the only thing keeping it on the page would have to be a huge CPC cap to compensate.
I guess Google could count haveing a phone number on the ad as a factor in its quality check.
Wouldn't it be better - on balance - to have this in the Desription? Much less effect on the serps, but more effect when (if) seen in the serps?