I write plugin for joomla that works with virtuemart and send sms to user after registration in virtuemart. you can use this code and change for other SMS gateways. this plugin send sms with farapayamak gateway. Programmed by Ahmad Balavipour in Joomir Site Design and Sitedar and hosting By Joomir.

Name of mobile field in virtuemart registration field is 'phone_2' and you can change this parameter to anything else.

Download here