BETA
If 2-way SMS has been enabled for your service, within the DID Management List page, all Mobile will have an option where you can activate 2-way SMS for the user using that mobile number.

Update DID screen with the Two-Way SMS toggle in the off position
Once the 2-way SMS toggle has been turned on:
Incoming SMS - will be delivered to the selected extension
Outgoing SMS - Any SMS sent out by the user/extension will use the Mobile number as the Sender ID.
Using 2-way SMS
Users can login to the portal and click on the User Portal icon.

Device list row with the User Portal icon tooltip highlighted next to a Standard device entry
This will launch the WebRTC and users can start sending and receiving SMS.