Screen turns off /locks off automatically in between a skype call.
Why does KDE 5.14 Plasma doesn't detect that I am in a skype call?
Does this problem exists with other application also ?

While you haven't provided a lot of info in your message, I have to assume that your power settings are kicking into while your are in a call. I'm pretty sure this is, at least I consider it, a problem across all the desktops. They only "monitor" for mouse and keyboard activity to determine whether Xscreensaver or Power settings should kick in. Not only is this an issue with Skype calls but it's also an issue when watching movies and TV shows on your computer. Maybe one day someone can program Linux to monitor that too.

There's 2 things you can do to keep it from occurring, as I see it.
1) Disable your power settings and always use the power button on your computer monitor. If you're concerned with leaving it on and getting screen burn in, you can install Xscreensaver. However this will automatically kick on during your calls and TV viewing as well. So you have to keep changing it from your screensaver enabled/disabled settings.
2) remember to move your mouse occasionally while you're on a call.