First Put iphone in lost mode, hacker will not be able to use the phone, he cannot erase ur data as well
Second u can track the iphone from lost my iphone section if he still has phone switched on. If its not showing up there then only police can help track it
Login into apple ID ,Put your iPhone on lost mode
you can track the phone through find my phone.
Importantly get help from local police and go to the area and try to retrieve the phone.
If these guys are pro and you are unlucky they might disassemble the phone and sell into spare market
If Stealer Is At Pro Level , Emergency Contact Or Replaced Sim Might Get Spam Sms Contains Phishing Url With Apple Cloned Site Asking U To Enter Device Login Pin And Apple Id Password.
Be Careful 
Mugged (robbed) from hand while on call
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First Put iphone in lost mode, hacker will not be able to use the phone, he cannot erase ur data as well
Second u can track the iphone from lost my iphone section if he still has phone switched on. If its not showing up there then only police can help track it