If a mobile number is assigned to a user business email address, let Teams sync this contact into his Microsoft Teams contacts. And when a user launch his/her Microsoft Teams app, ask him/her to register their mobile number to their email address and account. Hence users can launch their app using their mobile numbers and peers from different companies can chat to each others. It's the only way to compete with the coming WhatsApp for Business. Knowing that WhatsApp is the most adopted chat communication for businesses now because of its flexibility, wide spread and user friendly interface. and why not opening the app to sync with the whole phonebook like WhatsApp. hence creating a light Microsoft Teams for consumers.