No-phone-number-required, gratis and cross-platform VoIP application
I need to be available by smartphone for potential customers which generally all of them reside in a country different than the one I reside at.
Register-with-phone-line VoIP applications such as WhatsApp aren't good for me because I don't have and don't want to have a phone number based on the country of most of my potential customers.
I could use my current phone number with WhatsApp but potential customers bumping across "unidentified" phone numbers may feel odd or as possibly going to be scammed; furthermore, I myself try to move between countries and seek not to rely on just one phone number.
What may be a smartphone application which is gratis, cross-platform (Android/Apple/etc.) and non-phone-number-based (rather, at-least-email-based) with which I could easily interact with potential customers in chat and/or voice and/or video?