Gravity Forms SMS Add-on is a WordPress plugin by Devin Walker that sends you a text message every time a visitor submits a Gravity form on your site without needing a third-party API such as Twilio, Clockwork, SendFlow, etc.
This plugin uses PHP library to send text messages via the email-to-SMS gateways that many popular carriers provide. Sending messages with Email to SMS is free (although recipients may be charged by their carrier as with normal text messages).
The author notes “Please be aware of a few drawbacks to consider before using: Messages you send will come from your email address, not your phone number. You may also be limited to less than 160 characters, depending on the carrier. Finally, due to the nature of email-to-SMS (messages are sent as emails to the carrier’s gateway, which resends them as text messages) this method can be somewhat unreliable depending on your carrier.”