Great question. Phone numbers remain available because they're highly effective for digital ad matching, particularly on Meta.
Here's the thing... Many people don't update their phone numbers on social platforms even after changing or abandoning them in real life. The old number still lives in Facebook. So even when the number isn't suitable for phone outreach, it performs exceptionally well for ad matching and omnipresent marketing.
Common mistake: Users attempt to call a number, don't reach the person, and incorrectly classify the number as "invalid." But that same number could match perfectly in a Facebook custom audience and serve them ads. Different use case, different results.
Phone numbers are retained for matching purposes only, not for direct contact.