If you haven't reported it lost/stolen to your carrier, then calling functions would work as normal. If it's going straight to voicemail, that means it's either shut off or the signal is being blocked (a real possibility in a theft case). Factory resetting wouldn't stop calls if the line itself is still active.
If it is reset, Google now has factory reset protection. After the reset, you have to enter the Google account info of the person that was last logged into the phone. So unless they have your Google name and password, the phone is basically useless to them.
You can try the Find My Device app groom Google. If your phone is on and able to connect to the net it can help you track it down and I think remotely wipe it if needed.