The length of time for which your phone will ring is dictated by your carrier, not your phone.
Call your carrier and tell them you want the full 30 seconds (usually the longest allowed by most carriers) of ring time.
I hope that helps you. 
- If my response has helped you, please click "Options" beside my post and mark it as solved. Clicking the "thumbs up" icon near the bottom of my response would also be appreciated.
My Z10 was furnished by BlackBerry but all opinions are my own.
