X_NOTIFY_SMS_NUMBER_INVALID
Means
an SMS number is not E.164
Typical cause
the number smsChannel() resolved for a recipient is not +, a country code and up to 15 digits in all (E.164) — a local spelling a provider guesses the country of is a message sent to a stranger
Fix
store numbers as E.164 (+4915112345678) and return that from smsChannel({ numberOf })