If you're talking about the tome you hear when you're calling someone else, that's generated by the telco's central office, it has nothing to do with your phone. You're hearing the tone the central office sends to the calling party, whatever kind of line it is - cellphone, PBX or single-line wired phone.
I haven't heard of any US telco allowing the customer to select a ringback tone since the days of "the Bell System".