https://extensions.joomla.org/extension/extension-specific/virtuemart-extensions/send-sms-after-registration-for-virtuemart/
Send sms after registration for Virtuemart, by Ahmad Balavipour - Joomla Extension Directory
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...
send smsafter registration