This is a great idea and a very urgent need for Hubspot users in other countries outside the US. I think email is only important to Americans. In other countries, people tend to use phone numbers, Facebook messenger or other chat applications to communicate with each other, especially at work when the need for high-speed communication is the norm. For most countries outside the US, phone number is a better identifiable property than email. Imagine we have 100 contacts in Hubspot, these 100 contacts have 100 phone numbers, but only 15 contacts have email.
The phone number, not the email, is the factor determining whether 2 contacts are duplicated or not.