You’ll have to report it as lost to your server. I had mine stolen a few months ago, I’m with Orange and had a new phone and they let me keep my old number.
Here’s where it gets curious, I’m still with Orange on contract. But every month I’m having a text to my number from Virgin media saying thank you for my monthly payment and saying how many free minutes I have. I’ve never been with Virgin media so I’ve wrote to them, E mailed them etc to say whoever is using that phone and that number, it is a stolen phone. No replies from them whatsoever.