You are both wrong. You need a search warrant if you are attempting to access the drivers physical phone. You only need a subpoena to obtain cell phonereceived, how do you prove the vehicle didn't have legal handsfree equipment in place to facilitate a lawful telephone call?
There are far too many questions and it appears highly unlikely that the officer was doing anything other than gathering identifying information on drivers involved in a wreck.
Actually, you're wrong; a subpoena is required for civil lawsuits or to get witnesses to testify. A search warrant is for criminal investigation records which cell phone records would fall under; or to seize and search a physical phone. I've written and served hundreds of search warrants for phone records and (at least in the state I served) you won't get a judge to sign off on a search warrant for cell phone records for a traffic collision unless it's major injuries or death involved, i.e: felony criminal investigation.
