In this day and age, I don't find it odd at all that he would get 11 calls between 9 and 2.....at work. I think MOST 20 somethings get multiple calls throughout the day...and make multiple calls...WHILE at work. I also think there were likely numerous text messages--if he did that often. Nearly ALL of my friends spend TONS of time while at work on Facebook, Myspace, texting, or on the phone. See: Krista on Websleuths in the middle of the day ;-)
IMHO.
As for the odd circumstances at work that monday....
I'm speculating, But I think that this is the type of job where he isn't constantly monitored. I think he went to the break room for ONE reason....to make his presence known....to give himself an alibi. Once people SAW him at work he could leave--potentially--without being seen/noticed.
I find it INCREDIBLY odd that he would make up a story about being fired/the argument. How strange, knowing that his story would be checked out.