maybe that respirator sound was kind of a fluke--maybe he was still trying to get his bearings (it had only been 2 or three hours since he died)
Just speculating, but maybe he was able to reproduce the respirator sound because it is electrical. As you know, they seem to be able to facilitate communication better when they have some way to tap in through electricity, so he had both the phone to work with as well as the respirator sound.
I read some EVP experiments and the researchers use white noise for just that reason.
Another thing that occurs to me about the respirator is something that was told to me by a friend who has had many encounters with those who have crossed over. She was taught this by a NYC medium. She said those who have recently crossed over will use something familiar to us as a way of introducing themselves.
When my friend's dog (Crumpet) crossed over, her spirit came to visit me the following morning. When I saw Crumpet in the visitation, she was next to this long, thick, very ornate drapery panel. In her later years, she loved sleeping next to that panel and many conversations between my friend and I took place with her snoozing away in the room, next to this drapery panel.
I told my friend about this and that's when she told me what the NYC medium had said.
So based on this, I'm thinking that your dad used the respirator sound to communicate because it was familiar to you.
Now, your mom...I'm thinking on that one.
Cher