Hi some_random_dud, thank you for posting.
Sorry to hear of the issue with your new SIM whilst trying to make calls!
If you can please confirm a few extra details for us so we can look into this further:
1) Have you ever made a successful outbound call since receiving the new SIM?
2) Are you able to receive calls to the correct number via the handset?
3) is your new SIM showing as active and connected to a network?
4) Which make and model handset are you using?
5) Are you able to send/receive texts to the correct number via the handset?
All the best.
Molly