Bro,so you want to control only ringtone volume..? If so..then go to sounds and vibration and disable use volume keys for media..now you can use volume keys to increase ringtone volume..else there is mo option.. or Don't disable anything..while pressing volume key, click at drop down panel here you will find an options to control volume of media, ringtone, notifications and so on..
NOTE: BUT YOU CAN'T CONTROL SUCH OPTIONS FROM THE LOCK SCREEN..
