In another case I have followed, Missy Bevers, LE got a tower dump to identify all the numbers for a couple of hrs before the murder. That SW was the first iirc, they got the very next day. But of course it does not help much if it isn't a contract phone, however some cash phones can be traced to where and when they were sold and the video surveillance can be used, but I think that's a long shot.
I believe it is noteworthy they have not identified who came to pick them up. They said her sister dropped them off, and another family member was to pick them up. That they won't say who was there and what time they arrived is clearly part of the investigation. But why? I think therein lies a clue.