if you want to track the phone keep paying the bill and do not call verizon to tell them to deactivate the sim. keep it going , keep the phone alive, because as long as its turned on , it will report back.
if someone got a hold of your iCloud account then call Verizon i guess
i dunno about this dfu mode as far as activation lock goes. if its locked by a pin code, and someone dfus it, i think its still trackable.
maybe ill test it again later , ill dfu my iPad and see if i can find it
once you give up on ever getting it back, then you go to
www.icloud.com and erase the phone from your iCloud. DO NOT REMOVE it from iCloud, and leave your name and number asking for your phone back
then you call verizon and you tell them i need a new phone, please deactivate my sim
one thing i know for sure is if you deactivate the sim card, and its locked by a passcode. its not going to connect to wifi AT ALL to report back to apple, unless it connected to that wifi network at least once before.
its the same argument about iPods and wifi only iPads, it can't connect to free wifi like mcdonalds and statrbucks and click on i agree if its locked by a passcode, when it has no cellular connection.
if you deactivate the sim card in a stolen phone, you are basically given up getting it back quickly
if you have some proof that the sim card has been removed and thrown away, then yeah get a new sim card
FINAL TIP
=======
everyone should turn off control center access from the lock screen so the thief can't put it in airplane mode.