Originally Posted by dliesse

It's actually unlikely that AS is running out of flight numbers. I don't think they're into the 10,000 flights per day range, yet.

What's more likely is that there's a conflict with other airlines' flight numbers. The rules may have changed slightly since I was working for a different airline and learning everything I could about scheduling, but back then two airlines couldn't have the same flight number scheduled at the same airport at the same time. This was to avoid confusion on the radio. As a made-up example, we all know that airlines love to fly flight 777 to Las Vegas (or 1492 to Columbus). Can you imagine the on-air confusion to hear calls for United 777, Cactus 777, Southwest 777, American 777, Continental 777, Trans World 777, and so on all at the same time?

Even now ATC will warn if another carrier has a flight with the same number on the same frequency. Especially in the terminal area there's a lot going on in the cockpit, and quite often the flight crew picks up the number but not the callsign.