Starting today, callers in Florida must dial the three-digit area code before all long-distance calls. That means many calls within the 813 area will require an area code prefix. For example, 1-555-1234 would become 1 (813) 555-1234. Customers who forget will hear a recorded message advising them the call can't be completed. Phone companies say the move is necessary because they are running out of combinations for telephone numbers. In the Tampa Bay area, calls between Tampa, Clearwater, St. Petersburg and Tarpon Springs exchanges will continue to require only seven digits.