If your message exceeds 160 characters (70 for Unicode SMS) and you wish to use multipart messaging, you must include the maxsplit parameter with your HTTP API request.
This parameter specifies the number of times the SMS is allowed to split.
For further information on HTTP API parameters, please refer to the HTTP API Documentation.
For further information on multipart SMS, please refer to the knowledgebase article on Why is my message split into multiple parts?