Sending SMS messages from the desktop

We are currently looking at setting up our users to allow them to send SMS text messages from there desktop to our staff on mobile phones.

We do not want to use sites like Lycos etc as they are unreliable and not always instant.

We either wanta small program on the desktop or even better a plug in to our groupwise messaging system.

We know we either need to dial out or connect through our proxy server, but would prefer to be charged per sms.

Has anyone got a system like this running realiably?