Disable incoming calls overlapping current call.

This is a simple as it sounds, it's just so that if you are in a current call, incoming calls don't overlap the current call as this can cause at the least a hindrance as your current action with the current call has to be done all over again, and can even cause to mishandle both calls if the new call pops just as one is to click.

Of course, it should be an optional to either have it on or off according to the user's preference.