BlueCreek

It sounds a bit off, I have to admit. It's easy to see how it could be completely nothing, but at the same time how it could be an emotional affair at least on some level. You do need to be a bit careful though, because she is an ex-coworker and he's talking to her from work, so it could honestly be advice/chat regarding the type of business they are in. If that's the case, don't let him call you insecure though, that's an insult. People feel how they feel, and it's up to our spouses to acknowledge our feelings and do what they can to assuage them, not basically call us a name and laugh it off.

Still it is possible he is getting something from chatting with this woman. On some emotional level she could providing him something he's not getting at home. I'd try and talk with him and find out what he might be missing from your relationship. Don't place it in terms of an accusing "what does she give you that I can't?" but just tell him that he can talk to you about anything and everything, that you are open to him asking anything from you and that you want to be the person he calls or texts to from late at night in the office, may be even hint that you'd find it exciting and fun. If it really is nothing, he won't come down on you for acting jealous but probably jump at your offer to spice things up a bit with some naughty chatting, nothing wrong with that at all!

But there are two things I'd ask him for if I were you. First, I'd ask him to tell you all about this woman just so that you can understand more who she is, what they worked on before, and why he is talking/texting her so much now. He needs to understand that as long as you are in the dark, as long as he is keeping a secretive friendship going with a strange woman (out of state or not,) it is emotionally affecting you. The second thing I'd ask is that if her friendship is important to him despite your worries, he should at least talk with her on the nights he is at home instead of at the office. It's a fair request because he is alleviating your emotional stress because you won't be inventing things they might be discussing. Even if he is only talking to her about work related stuff, still, ask him to come home to make the call there unless it is some super time sensitive critical question, at least until you gain a level of comfortability with all this.

If he balks at either of these ideas, I'd start to be a bit more worried. Regardless of what Harry told Sally, yes men can have platonic friendships with women, but they won't be defensive or try and keep them from their spouse if that's the case. Remind him that his duty is first and foremost to you. Tell him that you are OK if he has a friendship with a past co-worker, but for your happiness and peace of mind, he needs to keep it out in the open while you are around. Tell him you'll make up for it by sending him erotic text messages, it'll be fun.