I want to integrate SMS service with the software I am developing, so that users could send SMS to their clients.
Really the question that you need to ask is, which SMS service would be easiest to integrate with C#. Most of them out there have Web APIs, which would be easy enough to tie in with a C# application.
A few example gateways:
http://www.mediaburst.co.uk/api/
http://www.messagemedia.co.uk/
Of course, to send SMS this way will cost money (usually in the form of credits with your chosen gateway). If you meant to ask how can you interact with a physical phone to send messages, well that would all depend on the phone, it's o/s and software available for it.
Perhaps you can get actual SMS integration by sending an email ('[phoneNumber]@[carrierDomainName]'). This is how I integrated paging into my EventLog in the past. The cellphone domain name for each carrier shouldn't be too hard to determine. Verizon's is vtext.com.
I'm guessing you want an API of sorts. Check out the following libraries:
http://twit88.com/blog/2009/07/31/c-sms-library/
http://www.topshareware.com/mCore-.NET-SMS-Library-(LITE)-download-42284.htm