What to do with this narcissistic Hoover attempt?

I’ve posted on here a few times about a narcissist I dated. For a bit of context. Some of the things he did were tell me my son is in the way, throw something at me when I didn’t want sex, and demand I stopped taking my sleeping tablet in case he needed sex in the middle of the night. I also work with this person. Same company but different locations so I don’t have to physically see him but I do occasionally have to talk to him on the phone. When I do have to talk to him, I am very cold distant and professional. I can tell this drives him mad. After things ended with him, he got back together with his ex-wife and this is his current situation.

So the other day I was out somewhere with my new boyfriend.. I gave my boyfriend a kiss goodbye and when I looked up my ex narcissist was standing a few metres away just staring at me. I knew in my bones that he would make some kind of attempt to talk to me over the phone at work and of course he did. He’s desperately trying to be nice to me again. So what do I do? One: completely ignore him? Two: pretend to be nice to him just to fuck with him (I know this is the wrong thing to do and I won’t but I would love to) or three genuinely bury the hatchet and just be friendly?