But the carrier who provides the service to advertisers to target messages and directly load them into subscriber inboxes can charge their subscribers lower pri

But the carrier who provides the service to advertisers to target messages and directly load them into subscriber inboxes can charge their subscribers lower prices because they have another customer, so your hypothetical ethical carrier will lose out to them. Hell, if they can open up the ad sales channel enough they could probably give away calling for free, just like a proper modern internet business. Does the existence of ad-funded free search engines mean the market naturally ought to produce spam-free search engines that steal all the customers?