you have to use speaker mode or use head phones so it does not lock the screen!
and better do nothing else while talking
Android marketplace( Google Play) shows Skype voice and video calling supported for Samsung Galaxy Y: Android-2.3.6( Gingerbread). But actually it only supports voice calls. The reason is that Galaxy Y doesn't have front camera and 832MHz processor( comparatively low speed). But I must say voice call quality and experience is nice. I am sharing details about voice calls based on my Skype voice call experience with 2G data network.