It's best to be honest in relationships as they can't be forced (or shouldn't be). Maybe the phone calls stopped as they just took up too much time, added to stress - or were/became an ordeal??
Perhaps you had far more to say and your son was mainly listening? I remember being quizzed by my mother (during the dreaded two hour calls) about the children's lives - with never a thought given to me. We never had a close, loving relationship, so I felt ignored, invisible, uncomfortable - and bored stiff.
It was just a failed attempt at 'let's keep in touch'. Surely, you don't want your son to ever resent the 'duty calls' like that, do you? You wouldn't want grandchildren summoned for an unwanted chat either.
Your ex-husband and wife (and DIL's parents) will have different circumstances and personalities, perhaps find travel easier, have more money to spend on holidays etc. - leaving you the odd one out?
So - don't stress about it, never expect life to be fair, just get on with your own life and accept being in the background. Do keep in touch, but accept that you don't have a close relationship now, due to distance and circumstances.