Nothing comes to mind right off which would be quick and easy.
Have you tried assigning a much higher Priority to the calls (Attendant uses 50 by default...you could bump that to 200, say, and leave chats at 50). That should route the calls first.
Of course, then you have Chats waiting in queue after all the calls have been answered if you have a lot of calls coming in.
To my understanding most call centers that allow Chats and Calls to the same queue tend to allow one of each in Utilization, so an agent can be on a chat and a call at the same time, or two/three chats and no calls (chats at 33%/max of 3, calls at 67%/max of 1)