Ringback tones, i.e. a song heard by your callers before you answer instead of a ringing sound, are a function of the phone company and available service packages for different phones.
AT&T has such a service, called Answer Tones, but the iPhone is not listed among the compatible devices.
just to acknowledge your post as being helpful. I wouldve at least marked your reply as helpful. Of course thats up to the og poster, but should be forthcoming. I didnt know what ringback tones were so thank you !