Telephone PIP review

I received a letter recently (addressed to me) to inform me that my son (20 years old) has his telephone review appointment on the 25th Sept at 11:30am.
I am his mother, carer and appointee. He had a telephone assessment in March 2020 and gave his permission for me to talk on his behalf so I assumed this was already on file, especially as the letter was addressed to me and didn’t mention that he would need to be there.
He was at work, I was ready for the call. They called at 11:10 yesterday and asked if my son was here, I said “No, he’s at work” and the woman said “Okay, before we go any further I just need to check with my manager that it’s okay to proceed without him. I’ll call you STRAIGHT back, would that be okay?” I said yes, that’s fine and explained that he gave his permission last time and that the letter was addressed to me and it didn’t say he needed to be here for the call. She said it should be fine but she just needs to check with her manager and will call me straight back.

Well, I waited and waited - all day. She never called me back. I called them just after 5pm and the man said he could see on my sons file what happened, she called but never called back and he said “They will probably call you tomorrow” I’m sorry but I’m not free tomorrow. I need notice and certainly need notice if my son needs to be here to give his permission.
It’s now almost 1pm the following day and I still haven’t been called back by this woman.

Has anyone else experienced this? I’m worried they will stop his money and say I didn’t participate in the call.

I’m so angry that it has been passed back to DWP as a “Did not attend”
I have screenshots of the number that called me that one time and the 1min time log.
No one else has ever called me back. The lady that sounded really nice who said she would call me straight back - never did!
I recorded this all too so I have that as evidence that I did in fact attend the telephone appointment. I’m just so angry at how they have lied. Saying I didn’t attend!