Definition of 900 number noun from the Oxford Advanced American Dictionary

 

900 number

 noun
noun
NAmE//ˌnaɪn ˈhʌndrəd ˌnʌmbər//
 
 
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a phone number that begins with 900. A 900 number is used by businesses that provide services for customers on the telephone, and a large fee is charged every minute Don't call that 900 number—it costs $4.99 a minute!
See the Oxford Advanced Learner's Dictionary entry: 900 number