It's a ̶̶u̶s̶e̶ pay it or lose it thing. AFAIK, typically applies to prepaid/pay-as-you-go SIMs, not on contracts.
For my case, it is that I have to make a 12EUR top-up every 3 months. The top-up credit will expire if I don't make another top-up on time. After a few months on zero credit, you get you incoming calls blocked. And after a couple more months, your SIM is de-registered.
Transferring your number is always possible, yes. As long as you're still the registered owner of the SIM number.