"Cactus" is the name used by US Airways in ATC communications. The name Cactus originated from America West airlines before they merged with US Air. After the merger, US Airways began using the "Cactus" name. They're not the only airline to use an unusual name; British Airways, for example, is "Speed Bird", Aer Lingus is "Shamrock", etc. Not sure about the flight number difference, maybe it just sounded like he said 1529 instead of 1549?. The flight number shouldn't have changed.