If you had Find My turned on, you can mark the phone as Lost, and even optionally request a remote Erase.
See: "Mark as Lost" and "If your iPhone or iPad is lost permanently or was stolen" in
Find your lost iPhone or iPad with Find My - Apple Support
If the phone is Activation Locked, and locked, and the thieves do not know your credentials (passcode, Apple ID password), the phone is going to be a "brick". Asking your carrier to blacklist it (by IMEI number) so that it won't ever get cellular phone/text/data service again will make it a "double brick."
Do not remove the phone from the list of devices associated with your Apple ID (as Apple describes in their last step) unless you want to clear Activation Lock, allowing the thieves to "make your phone their own."
Beware phishing. Apple Support will not contact you, offering to reunite you with your lost phone. (Apple doesn't get involved with lost phones.). Criminals may get in contact with you, pretending to be Apple Support. They will try to get you to give them your credentials, to take the phone off of the list of your devices, or to click upon a link, so they can clear the Activation Lock ("make your phone their own") and/or scam you in other ways. Don't fall for such scams.