I really feel for you, the constant messages and friction is a horrible way to live.
I now go back with 'i will be following the court order'. Any requests about what I am doing or why can't I do as they demand i just ignore, they do not need that information.
Stick to the court order, do not give in on your court allocated weekend as once you do it once they will keep pushing. Once it is a regular thing then court may say that as that is the pattern let's put it in writing.
You have a court order. Be strong wjth that. Although I know it is ridiculously hard to stay strong with the constant push back.
My ex recently refused to use the parenting app. Are you wanting to use a paid or free one? Either way I would message them a start date of where you will no longer be using WhatsApp and will only communicate via the parenting app. Name it, include the link. If it is Our Family Wizard or another paid one then give them a month to organise or something. I gave my ex a month, and then blocked him on everything. I also screenshot every message in that month and added it to the app so it was documented. When he finally logged on he was met with every message that he had sent me, and the few responses from me, which he was furious at.
It isn't okay to have to deal with this constantly.
With the calls, I know I felt like they are quite invasive... how do you feel about them? I found it best to prop my phone on the table, do it in the same room everytime (so they know as little as possible about your life and keeps it a safe space for you), and they only have to be 10mins if the kids are done by that point (my ex dragged them out for an hour). I also stopped speaking on the calls at all.
You aren't wrong, you're doing whst you can in a difficult situation.
Are the times and days for the ohone calls included in the court order? If they are, I also wouldn't be changing the calls to suit his time. They stay the same unless it is a special occasion or an emergency reason, he needs to be more organised. You and the two kids are able to organise yourselves to be available for the phone call, so he should be able to organise himself. If he isn't able to make the call then the call doesn't happen. he has then broken the court order not you.
Document everything, include the evidence where possible. Include the kids feelings out of 5 as that will support yiu in any further case.
Wishing you luck! These ex's that know and enjoy pushing boundaries just never stop!