I have heard a lot of people say that you should have no contact with your ex N and you should delete their number.
I agree that no contact is good, but sometimes this is not possible and also it can be easy to fall back into old habits and call them when times are tough or be sucked back in. So I suggest keeping their number in your phone and here is why:
Change the name of your ex N to "Narcissist" or "I am a Narcissist." They do not have a first or last name anymore! And also change their photo. Change it to a monster or a vampire, literally something you do not want to look at! This is key. No funny photos, something that you wouldn't want others to see or you wouldn't want to look at. You should not smile when you see this image. Over time you may, but not right away. If you want to connect your ex by name, I suggest using "Narcissist" as the first name followed by their real last name. "Narcissist Thompson" is a lot worse than "James Thompson."
I changed my ex N's name this way, so I know who is calling me and I associate her name with narcissism. I also changed her photo to a female vampire. Not a sexy female vampire either. A vampire that is angry, blood dripping and vile. And it kinda looks like her!
The psychology of this is that it forces you to see this person differently. You will not have floods of positive memories if "Narcissist" calls you and a horrible photo shows up. Not only will you not answer, but you will start to associate this image with them. This helps you change the way you see them in real life too! You will associate the new name and vile image anytime you run into them. They are not the beautiful, fun person you thought. They are a monster.
It is often hard for people with empathy to dehumanize others, but this will help. After all, who would want to talk to or be roped in by a person named "Narcissist" and with such a vile photo?
Below is the photo I used.
Good luck!