30006: Landline or unreachable carrier
Most often, the receiving phone number is linked to a landline. In this case, the reservation must be cancelled and be recreated manually from the smart locker graphic interface.
Attempt to send to unsubscribed recipient
When sending messages, you may see a response from Twilio about the message's From/To pair violating delivery rules. The Twilio API returns this message if you attempt to send an SMS or MMS to a recipient who has previously replied STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, or QUIT to your Twilio number. You won't be charged for this send attempt.
When a user opts out of receiving communications from a phone number that belongs to a Messaging Service, they're also opted out of all messages sent from that particular Messaging Service.
If you want to send messages to a recipient who has opted out this way, they must first text START or YES to your Twilio number to opt in again.