When my girl starts complaining on the phone and I am bored by it, I just say "Ah" "Hmm" "Okay.." every minute or so, so she thinks im listening. If she stops talking I say something that fits in with whatever she said "Those idiots" "So unfair" and if I have really no clue at all what she was talking about I go with "And what happened next?"
Took me some time to get it right, but it works like a charm. The most important thing is to make noises like "Hmm" and "Ahh" so she thinks you are listening. If you stay silent while she talks, she will go "What did I just say?" and you are in trouble.