I’ll admit it’s a little weird. But she IS MARRIED. Unless it continues and they start spending hours on the phone together (or he’s ditching you to spend time with her or on the phone with her), you need to let it go. A random phone call…or spurts of communication…are not to be that concerned over. If you need to talk to him about it, you could frame it in the context…”what would you do/say/think if I talked to my ex ________ (that I had a long-term, committed relationship with)?” If he would be uncomfortable for you digging up ex’s and rekindling even “just friend” relationships, then he needs to examine what he’s doing.