Let me make it simple. This was his work phone. The police and FBI already have the complete records of every phone call, message, and email, to and from the iPhone. Except for a few during work hours from his monster wife's known personal phone, they are all accounted for as work related for the San Bernardino County Department of Public Health. These data were provided from Verizon, the carrier, as soon as it was asked for. The only thing they don't have access to would be any notes there may be still on the iPhone or any iMessages that are sent encrypted to other iPhone or iPads. However, the evidence suggests that there is nothing on that iPhone except work related data. Part of that evidence is that Syez Farouk did not bother to destroy it.
These two terrorists maintained two BURNER cell phones which, along with their own two laptop computers the throughoughly smashed and dumped into a lake. These burner phone were off the shelf untraceable phones for which the police and FBI can learn nothing because they do not even have the phone numbers or even know what carrier they were using. They can get no call, message, or email information from the unknown carrier for the simple reason they have no linking phone number to even start to look for one.
Given the obvious conclusion that all the crime-related stuff was on those phones, not the iPhone, I don't even see how the warrant was issued in the first place.