Since the application has been updated to the last version, I cannot hang up the calls and also quality of calls has gone down, that is not usable anymore.
I have WP8.1, it is possible to somehow get version before 126.96.36.199 which is current now?
What is your phone make and model? Are you still experiencing these issues in 188.8.131.52?
Is there performance degradation overall to the app or just to audio codecs (which is the codec you are using)?
There is no way to install an older version of the application since it is done through Windows Live Store and only the latest version is available there.
Still next month Windows Phone 10 will be out and we'll have a brand new version for it.