That Apple phone belonged to the County of San Bernardino. The killers probably used other phones, which were later destroyed, to communicate with their terrorist associates.
There's no "probably" about it. The FBI has the shards of what is left of those destroyed phones, along with the remains of the hard drives removed from their laptop computers as well as their smashed laptops, that Farouk and Malik smashed with hammers before they went on their Jihad killing spree.
There is nothing probative they can get off of those shards. . . not even the phone numbers or carrier's name of the phones.