When the phone isn't doing anything, the volume slider, if there's only one, is usually the ringer volume - and it should say it is. If it doesn't, or isn't, have someone call you and adjust it when it should be ringing.
Make sure you've chosen a ringtone that you can hear, not something quiet or "none" or "silent".
You can use an app like
Ringtone Maker to increase the volume of any sound file - even to the point that it gets all distorted. (I use the sound of an old telephone bell - cranked up as loud as I can get it before it can't be recognized. I never miss hearing it, even if it's under a heavy coat. Not "pretty" but it works.)