First of all, I have custody of my nephew and niece and have for almost 6 years. Both parents had the right to call, at our discretion, as often as they liked. However the key here was at our discretion. When the judge asked me if I needed to set up a time for the calls, I said no for 2 reasons. First, I believed that being parents they would use normal curtosy and call at times that would be ok for kids their ages. Second, I always had the option to simply NOT answer the phone. So, I didn't feel the need to have a set schedule. The Dad calls 3 to 4 times a week, and the Mom has not called since May of 2009. I have basically told her to not call again if she can't be consistent. It was to hard on the kids for her to "come and go" from their lives. I do have permenant custody legally. They never see her either. They DO see the Dad about 6 times a year supervised and 2 times unsupervised.
Now, without more info on why the calls are disruptive, I am going to assume it is a couple of things. Maybe it is causing a disruption in your normal daily routine. So if that is the case, my advice is to decide what time of day is good for you to accept the calls, and suggest that the calls take place at that time. Simply don't be available to answer at any other time. You have the power there. If you need to just turn off the ringer on the phone. If it is a situation that the child is getting upset, then make sure to have the phone on speaker phone, (I still do this to this day to make sure that all conversations are approriate), and if anything is being said that you deem to be inappropriate, take the phone and inforn your ex that you can not allow the calls to contine if he insists on having conversations about that or like that. Make notes and keep them so if you end up back in court you can present your case. Also, make notes about your child's behavior before the calls and after. If your child goes to therapy or school, ask the teacher to keep notes of any different behavior or comments made about the other parent too.
Unfortunatly, our children who are from broken homes have been put in situations where they end up in the middle of some pretty bitter feelings. Don't allow your child to be a pawn in a game with your ex. If your ex is calling because he misses your child and just wants to hear his or her voice and speak to him or her to stay close, that's fine. If they are calling to simply check in on you, that is not ok. I also suggest having as little to do with the conversation as possible. If at all possible, answer the phone without even saying hello, but just pick it up and turn on the speaker phone and tell your child to say hi to Daddy or whatever they call him. Then you do not even have to speak to him. When they are done, take the phone and hang up. If he is calling to talk to you, you will soon find out because the calls will stop since you are not getting on the phone. If they ask your child about you, take the phone, say I thought you were calling to speak to your child, and then give the phone back to the child. I am assuming that it is Daddy that is calling, and not Mom. (sorry if it's the other way around) Also, if the visits are supervised, there must be a reason, so I would be sure to keep the calls short and supervised as well. I hope it works out. Just remember, you have the power to answer the phone or not. There is no way to know if you are home or busy. If it goes back to court, then you can get a court order for a better schedule and fewer calls that fit the schedule that works for you. :)