This is call messaging enabling the exchange of voice messages, a user can record and send a recipient a voice message if recipient's unavailable or for any other reason; as easily as making a normal call.
MTN VoiceSMS allows callers to move away from traditional typed SMS and instead record their own 30 second message with voice emotion and once sent your recipient receives an SMS notification, whether on MTN or other local network.
In effect you are sending an instant personalized voice message, without the need for the recipient to have an active voicemail box.