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
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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