Seems like there could be a bunch of different things happening at the same time. If for 15 years your DIL has been forced to spend your visits with you when she didn't want to, is it any wonder that she's deciding to stop? It may not be the best of manners, but it's very human. You say your GC spend a lot of time in their rooms when you visit. Do you think your DS or DIL is telling them to do this in order to send you a message? It could just be that they are older and want their own privacy. Your son distancing himself could indicate trouble in his marriage or a midlife crisis or trouble with his kids or he's uncomfortable not having his wife as some kind of buffer when you're there or issues at his work or his biorhythms are out of sync or, well, you get the idea. Please don't assume your DIL is behind his behavior. She may very well be, but if she's not and you make that assumption, you won't be able to address the problem since you mis-identified it.
Keep in mind that your options aren't limited to the status quo or total cutoff. Backing off doesn't always lead to a better relationship; sometimes it leads to more backing off and more and more. But sometimes it helps. I'd suggest calling less frequently and maybe cutting your visits down to under a week or only one 10-day visit a year. No matter how much you want your DS to pick up the phone, you can't make him. You can either be resentful that he doesn't call when you don't or accept it. Not easy to accept, I know, but healthy.
Also keep in mind that now that you have the idea that your DS and family are trying to cut you out, you will be inclined to interpret their behavior in that light. It's easy to jump to conclusions, especially when it comes to intentions and hidden messages. Try to be as objective as you can and separate what you KNOW from what you THINK.
Good luck. While there are many stories here about truly wretched family situations, there are also some great success stories. Don't throw in the towel just yet.