No contact, keep doing it or respond to ex?

It's been about 5 days since I've been doing no contact on my ex fiance. He's already reacted angry and passive aggressive in desperation for a response ("I'll just block your number I guess so I won't message you anymore...", etc.) And when that hasn't worked, I finally got a somewhat interesting text message saying "I hate to admit it but I miss you"... And he's then been trying to call me on Facebook messenger, call my phone, anything possible. I also noticed he's trying to re-add me on Facebook now... He's still sending me messages like "I'll leave you alone I guess"...

So my question, is what he is doing sincere or is it just another thing he is trying to get the satisfaction of a reply? Should I hold out with nc for an actual, definite renconcilation attempt? Because I've read that a lot of this behavior could also be breadcrumbs from your ex.
No contact, keep doing it or respond to ex?
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