That is the awkwardness that I am still not looking forward to, holidays. My MIL wanted to know what to do about Easter as well. I decided for her sake, as I am still very close to her,(and will remain so) that this impending divorce is hard on her too. Because my STBXAH and I are still on friendly terms, I'm just going to suck it up, and go this year. This is all very new, and I don't want to be the one to make it worse for everyone. Next year however, I may just send the kids and bow out as gracefully as possible. Yes, things are weird. No doubt. I guess it just depends on how close you are to his family and if you plan to keep them in your life, and just not the kids lives on some level, and what you should do until then, until things become "accepted".
I fear as well that my STBXAH will be contacting me frequently also, most likely in those lonely moments. Fortunately, he is more of a texter, so it makes it easier to ignore. I think as long as you know the location of your kids, I'd just turn my phone off at a certain time, or maybe you can tell him to text. For some reason that is easier to handle than a desperate voice on the phone. Good luck. I feel your pain!!