Told by whom?
If you mean by me then that was simply my understanding:
Perhaps I was wrong?
You should contact 48 support to check this with them.
Maybe they now charge for activation of replacment as well as new SIMs.
They used not do this but perhaps the policy and terms and conditions have changed?
But I can't find anything to that effect.
And this certainly suggests that the activation fee is only charged once for the initial activation and purchase of first plan:
€12.99 Plan
This €12.99 Plan is only available to new 48 customers who order their 48 SIM and purchase a €12.99 Plan from the 5th April 2022. A one-time activation fee of €12.99 applies with the first plan purchase.
Are you sure that you activated your SIM as a replacement and not a brand new SIM?
Is the phone number on the new SIM the same as the one that you were using with the original SIM?
Are you still using the same account and email address/password login to manage the new SIM?
To contact 48 support see the links in my signature below.
