ringtone volume

[yindi]

Whe battery goes below 30% ringtone volume goes to zero and I keep missing calls

[Alexander1]

Hi and welcome to the community! Since you're new please be sure that you have checked out our Discussion guidelines.

Do you have any power saving mode enabled? Go to settings > power saving. 

If you haven't, I would suggest that you try to boot your phone into safe mode to see if the problem may be related to any 3rd party app that you have installed in your phone:

http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008