I completely agree that integrating the logins would be difficult at best. The main reason I suggested the integration is that ATTCV offers this feature--from your CallVantage control portal, you can access ATT's billing site, and you're automatically logged in. The chief difference, though, is that your username (phone#) and password are the same for both ATT systems, and they're linked--change it in one place, and it's changed in the other.

I think a link to secure.voipo.com would be enough, especially since you're planning to use an in-house billing platform. Save the integration for that system.