isValidE164Phone
E.164: leading + or start immediately, country digit 1-9, then 1-14 more digits (15 max total). Whitespace and formatting characters are NOT allowed — callers should strip them first if needed.
E.164: leading + or start immediately, country digit 1-9, then 1-14 more digits (15 max total). Whitespace and formatting characters are NOT allowed — callers should strip them first if needed.