How to run web service automatically at scheduled timings

Web Services/Applications work on Request-Response model. So, you can not activate a web service itself to wake up and send SMS at certain time.

I would suggest to build a client application to call web service at a specific time.

You can do it in several ways but simplest would be write a console App (or even a PowerShell Script) to call web service and use Windows Scheduler to configure running at specific time.

There link can explain further:

http://msdn.microsoft.com/en-us/magazine/cc163821.aspx[^]

Scheduled Tasks Web Service[^]