Besides standard SMS messages, you can send flash messages using the Fully featured textual message

Flash SMS will pop-up on the user’s phone when it’s received. The message can be stored on the mobile phone and has a sender ID. In order to send a flash message, set flash parameter to true.

POST /sms/1/text/advanced HTTP/1.1
Host: sms.enet.co.ke
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
Content-Type: application/json

{
   "messages":[
      {
         "from":"InfoSMS",
         "destinations":[
            {
               "to":"41793026727"
            }
         ],
         "text":"Toto, I've got a feeling we're not in Kansas anymore.",
         "flash":true
      }
		]
}