Odds are, there was nothing worth knowing on that phone. Generally, Islamist agents prefer to use an ISIS-provided encryption app. But, since was an employer issued phone, they wouldn't likely install the app on it -- since it would be subject to inspection.
However, the couple was seen throwing items into a nearby lake the day prior to the attack. That was probably the phone(s) and laptop(s) that contained the information the FBI is after.
If it was an employer issued phone, wouldn't their tech dept. have the necessary code to unlock it? ...One would think so.......
I pay for my family's cell phones. That means that I have access to the online bill. The online bill shows every call made, every text sent or received, and who they were to/from.
Terrorists (at least those who are not complete idiots) would not use an employer phone for making their terror plans, or calling their terrorist friends. They would buy a cash-plan phone from WalMart, and use that.
Or even throw-away phones...