Yeah, they are fully interchangeable overlays covering the exact same Denver metro area! 303 is just the classic, original code from the 1940s, while 720 was added later when numbers started running out. If you ever need to track down a sketchy call from either code, you can use a lookup tool https://searqle.io/phones/720/ to find the owner's real name and city. Today, businesses and locals use both codes across Aurora, Boulder, and Lakewood without any geographic difference at all.