do i need a new mobile phone to use ringback tones?

No, you will not need to purchase a new phone to use the ringback tones feature.  All you need is a Cricket phone with a standard rate plan ($35 and up) and an active ringback tones subscription to use the service.  Ringback tones will not affect any ringtones on your mobile phone.  You do not have to repurchase your ringback tones if you change your phone but keep the same wireless phone number.