This is slightly impractical for non-critical communication for me because many of my friends are foreigners like me. Example issues:

- many people I talk to have phone numbers from different countries. Making such communication expensive.

- also, I suspect many people around me have not updated their WhatsApp number for a while (or have multiple SIMs)

- no one here optimises for SMS or call minutes. Some people will be on data only (either data only SIM, or Wi-Fi only)