Here is the quick installation process. You will need administrative rights for Zendesk Sell.

Go to your Onoff Business admin account and find the Integrations tab in the top bar menu.
Find Zendesk Sell on the dashboard and click it. Ensure you’re logged in to your Zendesk Sell admin account before you start the installation.
Click the 'Integrate' button under the Zendesk Sell logo.
Approve permissions to Onoff to be able to log all interactions in your Zendesk Sell account.
You will be redirected back to the Onoff Business admin account on the Zendesk Sell integration page, where you can select the numbers you wish to integrate.

Onoff Business