I'm sure that's so. Refelecting on my subjective experience when talking on the phone, I'm sure that my visual cortex gets involved. This isn't a matter of hallucination or profoundly altered state of consciousness, rather one of some kind of visual attentiveness different from what occurs in casual conversation.
But even conversation in the car can tend in this direction, depending, it seems, on the nature and subject of the conversation. Numerous times, I've rudely broken off conversation when driving and chatting with a passenger because it distracted from attention to ceveloping traffic conditions.
I haven't seen any mention of research on how phone conversations invoke visual cortex activity unrelated to what's coming in on the optic nerves but 100 quatloos says any such study would find that cell phone conversation while driving neurologically coopts part of the visual system that should be devoted to attentive driving.
GOoJF card for me: I don't have a cell phone.