After replacing the power cable screen flickers, but only during calls

I followed the guide to replace the power/sensor cable and everything apart for one minor detail seems to be fine now.

In the process the only other thing I did was to replace the battery.

The outcome is a fully functional iPhone 4S (tested cameras, WiFi, mobile telephony as well as internet). But whenever a call is received or placed, the screen which shows, among other things, the button to hang up, to enable loudspeakers etc, flickers.

The flickering only happens during calls (no matter whether incoming or outgoing).

What gives? Can anyone point out a specific reason why this would happen (and of course a remedy)?

For now I am considering this more of a cosmetic issue. During a call it rarely matters if the screen flickers (except when using the dial pad to enter further digits during a hotline call).