Is it OK to act the part of a deceased family member who an elderly patient, who is delirious/confused, wants to talk to on the phone?

Not in my book. I don't think it is right to lie to elders, even those who are confused. I would simply repeat that you are sorry that the person has died. From my long experience watching patients these things seldom work out well when the lies come into it. Not a moral judgement, but a practical one.

I would redirect or come up with an excuse.
"Mom can't get to the phone right now"
"Dad went to work"
"Dad had to take the car in for service"
"Mom went to the store."
"Billy is in school right now"

Use an excuse based upon the age of the person they want to talk to..but not the age they were but the age of when they are in your loved ones mind. Telling them "Billy is in school right now" will not help if in their mind Billy is 40 years old. It will work on the days when in their mind Billy is 12.

As said, this is a catch 22. Damned if u do damned if u don't.

I always liked this story.

https://medium.com/@SoldiersWhisper/he-needed-his-soldier-son-8f9bb20c88a

I think 'right' & 'wrong' fly out the window when we're dealing with dementia and delirium in elders, frankly. I've considered doing this myself a few times when my mother gets insistent on talking to one of her deceased siblings on the phone and demanding to know why they've 'abandoned' her. It's a terrible situation, really, and a Catch-22 for us AND for them. I haven't pretended to be her sister as yet, and have no plans to, but who knows what will happen down the road? The goal with dementia patients is to keep them happy and content at all costs. So if you think acting the part of a deceased family member will make your loved one happy, try it. To say it's 'not right' to do this is silly, in my opinion, b/c there is no more logic once dementia sets in!! A broken brain is at work now and we're desperately trying to give them a little bit of happiness here and there!

Good luck, whatever you decide to do! Asking an internet forum of people means you'll get all sorts of answers. Do what YOU think is best under the circumstances you're faced with.

I don’t know that there is a right or wrong here. It’s not something I’d encourage, I’d fear the person with dementia catching on somehow and feeling loss and deception. Maybe a quick, momentary chat and then deflect, but overall I don’t think it wise

I’m not so sure about it being “not right”.

When the end goal is securing peace, safety, and comfort for a victim of lost reality testing, it could be helpful temporary/interim solution for both caregiver and person in care.

No. Deflect and redirect with fibs but don't imitate the dead. That's just not right.