Why does my mother in law become really zoned out after talking to people on the phone?

I am not sure but I would guess it's because it requires so much concentration for her to follow the conversation and answer that it wears her out mentally because of her diminished capacity from the dementia. But that's just a guess.

This is listed under Alzheimer's & Dementia. Does MIL have dementia?

Does she have kind of a non-responsive look in her eyes? Does she not seem aware of her surroundings? I think that several kinds of dementia do involve brief episodes of "zoning out."

Does she do this at times besides after phone calls? Does she come out of it smoothly, on her own, as if nothing happened?

I wonder if the kind of concentration it takes her to have a phone conversation is the cause.

Have you mentioned this to her doctor?

Does your mom complain of any other symptoms like blurred vision, dizziness or sudden onset headaches? Does she stand up suddenly after her conversations? Does she become very animated or even upset during these conversations—like maybe a spike in her blood pressure? Does she respond to stimuli, like able to answer questions or respond when you call her name? I may contact her doctor and ask about this.

She has Lewey Body Dimensia. She will be totally alert and then she just shuts down as soon as she hears someone on the phone. No others issues. Sometimes she never really snaps out of it for a couple hours. She just will fall asleep and minimally responsive.