I just want to talk to my son...properly

My ex is very controlling and would only let me see or speak to my son when they said.
Long story short was we went to court and got a court order so he could have regular contact with me (bi weekly visits) and to talk via Skype once a week on an evening that was court ordered so he knew exactly what night we would get to speak.

Since court (which was only a few months ago) They have missed 3 sessions on Skype and my ex has now enrolled my son in a new club that takes place to same evening as our court ordered night to talk without consulting me!

They have given me the option to talk to him on the phone for a few minutes at the designated time but this is really disruptive as its right in the middle of his class and we don't get to physically see each other or have a productive conversation as he's distracted with everything that is going on in his club.

By letting him talk on the phone my ex isn't technically breaking the court order and when i try to get a new night to Skype confirmed they just ignore me. Any advice please?