My brother went through some very similar with his ex-wife. He is the custodial parent, and whenever his son was at his mother's (1000+ miles away) he would call his son every night. This wasn't per any custody agreement; he just wanted to talk to him.
This went on for years before his ex-wife decided she needed to change things. So she said my brother could call only at a certain time and that she would call her son when he was at my brother's any time she wanted. It was a completely weird power move, with her accusing my brother of trying to make her look bad by calling every day.
Eventually, they had to work out a formal agreement that allows free phone access to both, whenever they want. It cost them $$$ in additional attorney fees, and I felt bad for their son to be stuck in the middle. It might have been smoother had they worked out an agreement earlier.
I hope things work out smoother for you, your husband, and SD.