Is AUTHOR wrong in the post ? POST : Tldr: dad wants phone call every night son is with me but some nights I don't feel like it and I dont want to encourage son on nights he doesn't feel like it.

Background: My ex and I coparent very well for the most part. I have our son 4 nights out of the week. His father wants to talk to him each of those nights. Sometimes its not convenient, or one of us just doesn't feel like it. I have to be there monitoring because my son is 5 and will walk away from the phone, get distracted, or just not hold it right so his dad can see his face. Calls are between 5-15 minutes long. We use Facebook messenger as there are face filters that make it more fun for son.

They (dad and son) have a close relationship. Dad is a present, loving father which I am grateful for.

Ex is willing to call me the nights son is with him but I am usually busy. My son and I are very close and I dont feel like our relationship is negatively affected by not talking to him every night. In fact, I feel forcing him to phone talk sometimes makes him resentful too. The few times son has requested to talk to me I've always made myself available.

Tonight when ex called, my son asked for 30 minutes because he didnt feel like talking right then. Ex called exactly 30 minutes later and said because I didn't force the issue that son is walking all over me. He also requested that I be more involved in monitoring. Some nights I have walked away to brush teeth, do dishes or just my nightly routine. 
 
I'm becoming resentful of the fact that we dont seem to have an option to not have phone calls certain nights. AITA for feeling resentful? Are these phone calls necessary for my son's relationship with his dad? I don't get the feeling they are, I feel like I'm doing it more for his dad. 

Set me straight please, I'm not 100% sure whats right here.

Cross posted in parenting.