Your My 48 account page on the website or app will show the number associated with the 48 SIM. And calls to the number will come through on the phone with the 48 SIM installed. If there's a problem or delay with a number port then contact 48 support. See the link in my signature below.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.