My iPhone 4S switches off by itself when the battery is still charged 50% or more

I have a iPhone 4S, which switches off by itself (screen becomes blank) even though the battery has about 50% charge left. I am only able to switch it back on when the battery has reached about 5%.

During this dormant state, when somebody calls to my phone number, they get a dial tone as if the phone was still active, however I never hear the ring tone.

Despite my efforts to switch on the phone during this dormant state, I have not been able to switch it on (active screen state).