I would privately message her if you can't phone and ask outright why you're not able to be a bigger part of their lives - emphasis you'd like to provide practical support as much as possible in a way that only a grandmother can. It's important that kids get on well and have an active relationship with older generations - it makes them well rounded and balanced members of society. I would wait until you get a response before speaking to your son; he's likely to be protective of his wife or conversely want little to do with it all but if you've demonstrated that you've already tried to deal with this directly he won't have much choice in doing something about it. If you really want to be democratic about it the best thing to do would be to speak to them both at the same time, that way they can't deflect the situation onto each other.

Were you once an addict or have any mental health problems which may be factoring into their decision? If so you need to demonstrate that you're perfectly trustworthy and if need be have them supervise any sessions with your grandchildren if absolutely necessary. You're an important part of their life and you shouldn't be denied a chance to see them as often as possible. Best of luck.