The reason a killer would use the phones to call? To make it look like the girls were still alive for one (this what what happened in the case of my friend). I can think of several other reasons, like killer didn't have a phone and was calling to have someone pick him up somewhere, as if he had been with the girls and borrowed their phone and they were alive then. Then arranged to get picked up elsewhere in a half hour.
Again, unless there is proof the girls made the calls, if I were LE I would not assume that to be the case, unless of course they had phones with passwords like gte3$}re46 and nobody knew them ever. That is highly unlikely, but could be true.