Currently the behavior is dependent on the location of the recipient.
When sending a message to a landline number in the US, Canada and the UK:
Twilio will not check whether the number is a landline and will attempt to send it to our carrier for delivery. Some carriers (especially the ones in the UK) will convert the SMS into text-to-speech messages via voice calls.
When sending a message to a landline number in other countries:
The Twilio REST API will throw the error below, and the message will not appear in the logs and the account will not be charged.
"code": 21614, "message": "To number: +86XXXXXXX, is not a mobile number", "more_info": "https://www.twilio.com/docs/errors/21614", "status": 400
Note: If you are attempting to send messages to Twilio SMS-capable geographic numbers, Twilio would allow you to do so.