Calculate the number 7551

A man awoke one evening to discover prowlers in his storage shed. He immediately called 911, gave his address, to report the prowlers and possible burglary. The operator at the other end said "Are they in your house?" He said they were not, only in his storage shed in back of the house. The operator said there were no cars available at that time. He thanked the operator, hung up the phone and counted to 30 and called again. "I just called you about prowlers in my storage shed. Well you do not have to worry, as I just shot them all dead!" Within seconds there were 3 police cars, an ambulance and fire engine at the scene. After capturing the prowlers red-handed, the policeman asked the caller, "I thought you said you had shot them all!" The man answered, "I thought you said there were no police available!"