I’m starting a small company near Denver and noticed many newer businesses use 720 instead of 303. Is the older area code basically unavailable for new registrations these days?

You can definitely still get a 303 number for your Denver startup, but it's a bit harder because they are mostly exhausted. Standard carriers automatically hand out 720 or the newer 983 overlay numbers for new lines now. If you want to check the reputation of a 720 number you've been assigned or are considering buying, checking a lookup site is a smart move to ensure it hasn't been flagged for spam. To get a classic 303, you usually have to look into business VoIP services that specialize in transferring or hoarding pool numbers.