To get SMS to work, you first need to have email working. Having done that you can proceed with the SMS setup discussed below.
Two email-to-SMS gateways are supported out of the box:
To sign up, go to: http://www.smsglobal.com/en-au/solutions/signup_page.php
SMSGlobal allows you to send a small number of test messages for free after you register.
Now use Administration|Organisation|SMS Configuration: SMSGlobal Email2SMS
Clickatell allows you to send a small number of test messages for free after you register but sends a preformatted SMS until you pay.
To sign up:
After signing up, logon and click the 'Manage My Products' tab. Then click 'SMTP [Email to SMS]' link the click "Submit and Get API ID". The API ID is needed when configuring OpenVPMS.
Now use Administration|Organisation|SMS Configuration: Clickatell SMTP Connection
If you want to use another provider, then you should be able to do so using the Generic Gateway. You will need some knowledge of the gateway message format requirements and xpath expressions.
See Administration|Organisation|SMS Configuration: Generic Email Gateway
The SMSGlobal and Clickatell configuration screens hide the underlying detail, and indeed it is possible to configure an SMSGlobal or Clickatell gateway using the Generic configuration screens. Similarly, it is possible to create a tailored screen for another vendor. If you want to do this see here.
Having configured the sms gateway, you now need to set the practice to use the required gateway by using Administration|Organisation|Practice. Remember that you also need to set up email for the practice.