Are you sure the below line is correct?
sURL = "http://ars-sms.ir/API/SendSMS.asmx?Username='" + Username + "@Password=" + Password + "@TO" + TO + "@TEXT" + TEXT;
1. Can you call the service using HTTP GET?
2. Why do you have a single quote before the Username variable? Is this really needed?
3. Why are you using the @ character to separate your parameters? Is this required by the ars-sms.ir site? Normally you would use the & character.
4. I don't see an = character between the keyword and value for the TO and TEXT parameters.
If you don't know why or have an answer to the above questions you may want to try replacing your URL with the below...
sURL = "http://ars-sms.ir/API/SendSMS.asmx?Username=" + Username + "&Password=" + Password + "&TO=" + TO + "&TEXT=" + TEXT;