Agreed about the idea that there is a rule for how often and when you call extended family or parents, but in this case I think it was partially the OP's fault that they expected that. I believe that's already been addressed, but I'd say that a change of "rules" is in order.

OP: I thin that they need to understand that at this point, having a scheduled time/day for phone calls is unrealistic at best and if there is to be any kind of schedule, it should be flexible as you do not always know what the day will bring. Obviously you cannot require them to be flexible, only stand on your rule. My suggestion would be if you have to say a date and time, say "when my schedule allows" and add that to it. If not a schedule just let them know that you will keep in touch when you are willing and able to do so. Leave it at that. Do not bend and just let them learn to accept it. If you are strong on this, they will spit and pout about it at first but continuing to stand on this, they will eventually give up and accept or at least grumble quietly to themselves instead. I think this is key to your establishing your independence!