You mean you're trying to port your number to 48 but don't have your old carrier's SIM to receive the porting security code?
You might need to get a new SIM from the old carrier first to complete this process.
But contact 48 support in case there's some other way.
See the link in my signature below or email them on firstname.lastname@example.org
Hope this helps.
Great - that's good to know that they can do that where a new user doesn't have their old SIM for some reason.