Well,
Without knowing what I was doing, I went into NC as soon as I found out my ex (who technically dumped me, but I said the words) was seen flirting with another girl the day after we broke up despite him telling me he wanted to be friends. I was hurt and I wanted to protect myself. I managed it, then he messaged me 10 days later, it was a silly message, joking, I had nothing to say to him, so I ignored him for another 12. I finally came round and messaged him, nothing to do with our relationship, just an email saying all the best of luck in the future and other advice. He didn't reply, so having spent almost a whole day composing the email, I messaged him to read it. He replied not long after. Then there was silence, I wrote a reply to his message, and he did not reply. Then I left that too, a few days later, he was celebrating his birthday and I just wished him happy birthday and he replied. I know I should have left it at this, but curiosity got me, I replied once again and we talked a bit, me opening up a lot more about my life than he was. I was starting to sense that this guy was no longer into the friendship, so here I am, as of yesterday after waiting 10 days for no reply, I have begun NC. I actually begun it before reading this topic. I wish I had just continued NC, and as you wrote, if they want to hear from you, they know where to find you.
I think it's an ego thing, or he's found someone else. If he has found someone else, the anger from that is like petrol to my 'I'm so over my ex machine'...
I'm too nosey, that's my problem!
Btw, I know nothing has happened to him so he is intentionally ignoring my messages.