You should butt out of the relationship between your parents. They can work things out for themselves. Telling your mother about such pecadillos is only going to hurt the whole family.

And is there a difference between phone sex and, say, flirting?

As painful as it is to have overheard what you did and know what you know, you need to be quiet about it. It belongs to your mom and dad and there is nothing you can do, but be supportive.