Do you want to send SMS from your Windows Phone 8.1 App ? . You can use the ChatMessage class in Windows Runtime to do it.
Sending SMS from a Windows Phone 8.1 App
Here’s a code that demonstrates how to send SMS from a Windows Phone 8.1 App .
ChatMessage sms = new ChatMessage();
sms.Body = "Test SMS";
sms.Recipients.Add("11233");
await Windows.ApplicationModel.Chat.ChatMessageManager.ShowComposeSmsMessageAsync(sms);