<p>I have a scenario where I need to make a call to a telephone(landline/mobile)  or send SMS to a particular set of users only using ASP.NET and C#. The web application is not a mobile application.</p>

<p>How do I go about doing these? What typically would be the hardware requirements? I would be <strong>extremely grateful</strong> if you could provide me with pointers and reference websites.</p>

Not sure if you're looking for hardware solutions to automate yourself, or external services. However, I've used BT's
Web21C
pretty extensively.
They have an excellent .Net API and a host of functionality. Their pricing is the best in the UK, but might fall down with US SMS, which is obviously cheaper - there are plenty of other SMS API providers though.
What BT do offer, which is rare, is an API interface for automating call dialling, conferencing and managing call flow.