No, mom and dad worked out an agreement which seemed reasonable to both and considered the needs of the child. Now mom only seems to have issues with stepmom, and they seem more personal between the two of them than anything else. I think the best thing OP can do is try to keep the original agreement (phone call with no skype if computer access is not available seems fair), keep the child in mind, continue to try be be reasonable, and ignore any issues that mom tries to stir up if they are not reasonable. Dad too. There is no need to engage in any conversation where mom is being harrassing. Hopefully mom will get the picture and discontinue that practice. Either way, OP doesn't need to acknowledge it.
It might be a good idea also for dad and OP to keep a log/record of communication between mom and son.
I can't believe anyone would say that mom should be able to talk to kiddo "whenever she wants" regardless of what kiddo is doing or the family is doing. Returning a phone call is not that big of a deal.
Put myself in her shoes? Any parent, myself included, who spends long periods of time away from their children, is in her shoes.
And just because dad/stepmom doesn't immediately hand the phone over when mom calls doesn't mean they'll "lose." My ex actually tried to argue this same issue with the judge. She is the one who got an earful from the judge.

