You can use a song as your phone ringtone, in place of one of the ringtones that come with the phone.
Touch and hold a song in the Music Library list. In the Context menu that opens, tap Set as ringtone, then tap Phone ringtone or Contact ringtone.
OR
On the Playback screen, tap the Menu Key , tap Set as ringtone, then tap Phone ringtone or Contact ringtone.