If you’ve lost your phone, you should more concerned about what you’re going to do about it, rather than what someone else can do with it. I’m assuming you had already registered it with an account on the iCloud website and set it up to be tracked. Take one of the following steps:
1) Attempt to track the phone — if you’re ready to invest in that operation.
2) Login to the iCloud website and erase your phone; then buy a new phone.
3) Forget about the lost phone and buy a new one.
Option 2 would be the best. Also, make sure you change your iCloud account password, from a secure system. For assistance with erasing the phone, you can get the details either on the iCloud site, or else use this link: