I have purchased a O2 prepaid S sim card a month ago and it ends today. I am trying to recharge it so I can keep using same number

So my first question is, what happens if I do not recharge? I heard that my number is valid for 6 months. Would it be possible to receive a call? Maybe make a call?

Second, someone has introduced my a third party website and also I found that I can recharge a credit via paypal or post office. However, I am not sure how this works. The unit was 15, 20 and 30 euros. Are they indicating specific amount data or do I use this credit in different place?

Lastly, when I checked my O2 app, it indicated I am subscribed to prepaid S but I received an email saying my balance is not enough to extend it. If I recharge credit via paypal, would it extend through credit in my account?