Problem with answering calls

[Texas Ranger]

I have a TyTn II and am very satisfied with it. However I have an annoying problem and would appreciate any help to fix it. When I have an incoming call, according to the instruction manual I can answer either by pressing the "Call" button (the one with the green telephone symbol) or by touching the softkey for "answer" (or the area on the touchscreen where "answer" is written). In practice, only the second method works. I have lost calls by insisting on keeping the call button pressed instead of using the softkey.

The call button works just fine for accessing the phone dialer, so it is not a hardware problem. Is there somewhere in the registry where the call button has to be mapped to answer incoming calls?

I live in Italy and use TIM as a provider. This should not influence things at all.