Q: If a phone number has to be registered and validated, why not use that as the unique ID, instead of creating a separate VPA?
For comparison, in Singapore, the local UPI-like "PayNow" network uses numbers as IDs, meaning you can easily send money to anybody in the system (these days virtually everybody) without needing to know their bank. You can also transfer to any Singaporean company or organization via their Unique Entity Number, which is an existing company/org ID assigned at formation that includes a checksum.
For comparison, in Singapore, the local UPI-like "PayNow" network uses numbers as IDs, meaning you can easily send money to anybody in the system (these days virtually everybody) without needing to know their bank. You can also transfer to any Singaporean company or organization via their Unique Entity Number, which is an existing company/org ID assigned at formation that includes a checksum.
https://abs.org.sg/consumer-banking/pay-now