I'm new to using Ringio and ZenDesk, but my guess for a nice integration is that the phone call could go to support and if nobody picks up, a voice mail message can be left by the caller, which gets sent as a .wav or .mp3 attachment to the support ticket. 

Also, an added feature would be that the caller (if registered via the phone number they call on) would get a reply support ticket ID based on their phone call.

I'd like to see a ticket created automatically in Zendesk upon a call, and the information about the caller populated and entered.