If you send multiple SMS messages to the same user in a short time, Twilio cannot guarantee that those messages will arrive in the order that you sent them. While we will send the SMS messages you pass to us in the order that you’ve queued them, the SMS messages are delivered individually with no association to each other. The order of delivery depends on the carrier and the receiving mobile device.
Fortunately, most mobile carriers in the world support message concatenation. This means that an SMS message can be up to 1600 characters long, and Twilio will include data in that message to help the receiving device re-assemble it into one single message. For details about message concatenation and segmentation, see What The Heck Is A Segment?
In the rare cases where you may be sending long SMS messages to a mobile network that does not support concatenated messages, Twilio will automatically include a page reference at the beginning of each message, for example 1/3, 2/3, 3/3. This will help the user understand the intended order of the messages, although delivery order is not guaranteed.