Scenario 1: The person they hired committed to a start date and then didn't show up. I was working temp, and was due to be replaced by a permanent hire, and the new hire was a no show. No call, no email, no nothing, they simply disappeared. Most likely they were working multiple leads, and a better one came through.
Scenario 2: The person they hired proved to be a goofball. I had an interview, was told I would know my status the next day, the employer told me he'd hired the other person, thank you very much. Next thing I get is a message saying, hey, can you start tomorrow? Turns out the other guy made a mess of his first day at work and they told him to get lost.
Scenario 3: One or more existing employees stomped out. I had been having a progressively worse time at a job, and I realized they were assigning me 5 people's worth of work. They had no idea how overloaded I'd become, but I was getting nasties from my manager indicated that something had to improve. I went into HR on the Monday after I got my paycheck and told her "I'm done - Bye".
If they're not responding to calls, they either changed their mind again or they're in a mad scramble to find bodies, and can't get off the phone. This definitely has the flavor of an emergency on their part.