There's no reliable way. You'll find code online that tells you to look up SBFormattedPhoneNumber, and sometimes that works, but not consistently. Mostly it's people who brought their phone number over from another cell carrier. In those cases, you get a phone number, but not one for the current phone, and not one that's guaranteed to be valid for anything at all (it may not even be unique).
I have no basis for saying whether this is a few people or a lot, but it's an undocumented key that's inherently unreliable.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…