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21635: 'To' number cannot be a landline

ERROR: 21635

PROGRAMMABLE MESSAGING

ERROR

Description

You have attempted to send an SMS with a To number that is a landline.

Possible Causes

The To number you provided is a landline number.

Possible Solutions

Confirm that the number you are sending to is not a landline, using the Lookup API.
Include ForceDelivery API parameter if you still want to send the message to this phone number. Please note that this does not guarantee the recipient number will be capable of receiving the message. Some international carriers may have a text-to-speech feature that converts the SMS to a voice call.