Happygran1964, I can understand how you are feeling, but I really do think that maybe you're reading more into this than what it actually is. I'm in the same situation, and I've not seen my little GD for several months due to Covid/flight restrictions.
We all know it's a fact that daughters mostly gravitate towards their own mothers, rather than their MiL, so it can only be expected that your DiL sees/facetimes her own mum regularly. I have a good relationship with my DiL, but I wouldn't expect her to facetime me. As others have said, that's really up to your son to make the effort to facetime you ... or perhaps you could take the initiative and facetime him occasionally?
I know for a fact that my DiL facetimes her parents more regularly that our son contacts us ... I think that's a basic difference between sons and daughters. However, as time has gone on, our son has started to voice his concerns that our little GC isn't going to remember/recongnise us, so he's actually started to facetime us a couple of times each week. I'm very conscious of the fact that they have busy lives, so I tend not to try to call them too often, as I don't want to become that annoying grandparent, calling them when it's not convenient. And I don't get offended if they don't answer my call on the odd occasion that I do.
So, my advice is, it's completely natural for your DiL to call her own mother/father, and not call you. You shouldn't expect her to, regardless of your relationship with her. It's completely natural for you son to be lax in making contact with you ... sons are generally not as good at it as daughters! Try to facetime your son (but not too often!), and just ask how things are going with them, but don't expect to have a big long converstaion, and time chatting to your grandchild ... it simply won't happen! And like someone else said, if he doesn't answer, just follow up with a quick message to say you just wanted see how things are going with them all, and maybe he could facetime you when it's convenient, as you'd love to 'Hi' to your little GS!
Our GD is 2, and we're lucky if we get to see her for a full minute ... but she's always happy to blow us kiss and wave goodbye, which makes it all worth it!