It's possible to send SMS or text messages using Knack or using a third-party service like Twilio with your Knack app.
Using Knack Only
Sending an SMS in Knack is possible but will take some setup and manual input. Additionally, sending the text would work more like sending an email.
Most cell carriers have a standard nomenclature that they use if sending a message to a phone number from your email client. For example, to send an email to a phone number whose cell carrier is AT&T, you would need to use the following email format: [phonenumber]@text.att.net.
Example: 6028675309@text.att.net
You could build out your forms to capture the phone number and cell phone carrier. You can even go as far as using a text formula to combine these two fields. Either way, you would ultimately need to manually input this new email address into a new email field. You can then use this new email field to send notifications along with the email notifications.
Using a Third-Party Service
You can integrate with a third-party SMS tool like Twilio via Zapier. Zapier makes it easy to set up automation between Knack and other third-party services.