Hi @amymcn sorry to hear you've lost your phone! You can do this by going to the Change My SIM page on the 48 website - you can block your lost SIM there, and while you do that you can also replace it with a new SIM that will have your existing number on it.
When you block the SIM, service on it will stop immediately, and it will take 2-3 days for your new SIM to arrive.
For detailed instruction on how to do this check out the "I've lost my SIM (phone)" section in this FAQ.
I hope that helps! Can you post back to say how you get on?
