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.
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.