From SMS Wiki
How to Get Message Delivery Confirmations Using SOAP
First, you need to get token and signature (see earlier discussion oh how/where). When you send a message (using Send_Plain, for example) you will receive MTID (message or tracking id). This is a unique number assigned to each message. When you send a message you can inquire about its status (no more than every 2 minutes) using the GetDLR function.
In return, you will receive status of the message.