Welcome to the community ​@Waldous, even if it's for something as this  I understand your number isn't working with KPN but its also not working with your Youfone sim card? That's weird, since ‘nummerbehoud’ where you switch providers while keeping the same number, means that your phone number stays active with provider 1, until it's activated on the sim card of provider 2.

Did you ask Youfone to allow the transfer before the end date of your contract? Or did you END the contract? f you reach the end date of your contract, it does not mean you stop receiving service, it just means your no longer obliged to stay with a provider and are free to go somewhere else or you're allowed to end the contract completely. Since KPN did not activate your number, it should remain active with Youfone, unless you completely ended the contract. If that's the case, it's a bit problematic. It's not possible to transfer a number that's not active anymore, so I really hope Youfone did not end the contract completely. I would strongly suggest to contact Youfone and ask them what's going on with your subscription, since this is something KPN can't check for you so we won't be able to tell you if your number should still be active with Youfone.