You're almost at the finish line. 

The last part of setup now that your brand and use cases have been approved is to assign phone numbers and users. The phone numbers you have in your account can be used for sending and receiving messages as well as voice calls. To add numbers, go to the Phone Number section. Adding Phone Numbers to your account

First, select phone numbers for messaging use.

Name the message box- i.e. Support, Sales- for easy identification.

Then select the Users you want to have access to sending and receiving messages.

You can repeat this for multiple numbers.

When finished, press Save.

You'll now have access to the Messaging section of the dashboard. Sending and Receiving messages in the OnSIP App

NOTE- After adding a message box, it will take approximately 15 minutes to sync the number to the use case. Until the number is synced up, messaging will not work.