how can I call from Apple Watch Cellular to its Paired iPhone, over cellular, when iPhone is away.

If you have the Cellular + GPS version and the Cellular option is activated, you can be many miles away and it will work.

If you don't have the cellular option activated then you either be on wifi calling if activated and or connected by bluetooth to your iPhone. Bluetooth has a limited range usually no more than 45 feet (15M).

If you're talking about calling your own phone, you don't.

You can't call yourself.