Gamma, trapped in their burial, wanted to surveil Bob and Sally, and maybe influence what they did. Rather than breaking free of many layers of concrete, gamma used their world simulation to predict exactly what Bob and Sally said.

Gamma had practiced predicting humanity immensely in its conflict with us, and was quite confident it was correct.

Gamma want to influence the outcome of the conversation. Since gamma had participated in their own burial, they had left patterns in humanity to meet their plans for our behavior. Still, gamma was in the habit of continuously tweaking this behavior, to further refine their models and plans.

Gamma decided that, rather than tweaking the phone call, they would observe the result of it untweaked, and use accommodating any needed changes afterward as a further, more powerful, learning experience.