Possible Causes
The recipient opted out by replying with "STOP" or another opt-out keyword.
The number has recently been reassigned to a different user.
Possible Solutions
Ask the recipient to opt back in by texting "START" or another opt-in keyword.
Ensure consent before sending messages.
Is this billed?
No, this error is not billed by the carriers
Are contacts that return this error marked as “Opted Out”?
Yes, contacts that return this error are marked as “Opted Out” in your Textla contact list.