What was his deadline for you to call?
If you haven't heard by then, call 15 minutes prior to the deadline and say something like...
"I realize how busy you must be. I've already left a couple of messages, but we're coming up on the deadline (mention the time of your call) and I wanted to let you know that I am still very interested in further discussion about this opportunity."
Tomorrow is the deadline. And I did mention that in my last message - explained what the letter said.
But if I don't hear back by the end of today, tomorrow morning I'll leave a message that says basically what you just said. Sounds reasonable.