Some carriers retain texts for up to a year, others for a few days. All of them keep the meta data for a long time however (which is why they could place ZA and SA's phones at a particular tower). That data would include who called/text them or the reverse. So, that information would be known, and if they were communicating between each other as one might expect if the crime happened as claimed, there should be a record of that. I have not followed the trial live, but I expect that the relevant state witness would have revealed that information when giving evidence, since it would be clearly relevant.