I guess my response would depend on if you are in the process of a divorce or just in the middle of separation. If it is divorce then you both are moving on and it is just paper work that is stopping you guys from being married. If it is separation then I think it is a huge sign to who he is" if" he is in fact seeing someone. I think your gut is right... if it was just a friend then why would they not simply leave a message with you when you answered? No matter where you are at in the relationship it still is hurting you and that is understandable... you have been married for a very long time.
Time to focus on you and what you need. The less time and effort you spend on trying to figure him out the better.