Possible Causes
The number is a landline or the carrier is unreachable.
Possible Solutions
Use Lookup to verify if the number is a landline. If so, use a different phone number type.
Is this billed?
Yes, this error is billed by the carriers
Are contacts that return this error marked as “invalid”?
Yes, contacts that return this error are marked as “invalid” in your Textla contact list.