Site24x7 offers alert notifications via Email, SMS, Phone, and IM channels. The alert settings can be customized by logging to Site24x7 and navigating to

Admin > User & Alert Management > Users & Alerts > (Select the user's name to customize) > Alert Settings

If you wish to opt out of your preset "Phone" based alerting mode and go with only SMS based alerting, you need to uncheck the "Phone mode" provided and click Save.