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. A message record will be created in your Messaging Logs and Insights.
"code": 21635, "message": "'To' number cannot be a landline", "more_info": "https://www.twilio.com/docs/errors/21635", "status": 400