The other point of view:

My stbx and I share 50/50 custody week on week off arrangement and she calls everyday that I have the children while i try to call every few days would say average is 2 times a week. My reason is simple I know how the kids start to act after she calls...they start up with the 'why don't you live together anymore" "What did we do wrong" etc etc etc. She usually calls right before bath time and I spend the next hour reassuring them they are loved by both of their parents but that we just don't live together anymore, that they did nothing wrong and none of this is their fault and neither she nor I are going anywhere and leaving them etc etc etc. Before she calls during the week and on weekend neither of the kids do anything but act like kids. It also bears mentioning I would never discourage them from calling her if they wanted to and have dialed the phone for them numerous times (it's usually to brag about something that happened at school) so its not like if they think about it and want to talk to her they feel they cannot. The only time it really bothers me is when she decides to call them 2 times a day, once in the morning while I am getting them ready for school/daycare and then when we get home from work.

It annoys me that she does this when I have the kids yet she lets them spend the night with her mother atleast 2 nights a week during 'her' time and I know she doesnt call them on those nights. She isnt doing it for the kids she is doing it for herself.

To be a little brutal and honest it sounds like you are doing it for yourself and not the kids as well.
"I go through seperation withdrawals""I am used to etc etc""I want to hear their voices" I I I. He gets them approxiately 2 and 1/2 days eveyother week and you want to intrude on his time with the kids every other day? Would it be so wrong for the kids to not have to be reminded that their parents couldn't keep things together even if it was just for their sake?

just my 2cents worth and will probably get slammed because I have the wrong chromoson make up and therefore do not understand.