Sending SMS inside Interactions
Go to Contacts -> Interactions
Click on the conversation in the left sidebar
Click on the SMS icon

Send an SMS:

NOTE! Voice messaging is not possible via SMS inside Interactions at this time, only text.
Request your own dedicated SMS number
To be able to send and receive text messages using interactions, you’ll need to request a dedicated number.
The first time you access SMS inside interactions, you will be required to request access to the SMS feature:

It has different associated costs depending on which countries you’d like to send and receive messages from. You’ll be informed throughout the process depending on your specific use case.