It's been 5 yrs since I broke up with my ex. The fact I left him always made me feel guilty. Yet I found my Mr Perfect who has always been there for me and who loves me tremendously. But a few months ago, my ex (who NOW has a girlfriend) has got my number and is texting me. I pleaded him to stop and just get over me. He does stop for a certain time but continues miscalling me and I let him (as sometimes I still feel guilty for having broke up with him). Once his girlfriend even called him and told ME to stop calling my ex ( he told her that I was the one who called him) and given that I didn't want him to get into troubles, I told that girl that I won't call him. And as usual he did stopped for some weeks but restarted miscalling me. I don't know what to do, I even threaten him that I'll tell his girlfriend about him miscalling me. However recently when he miscalled me, I didn't say anything. The biggest problem is that my boyfriend doesn't like my ex at all and if he ever he knows about this miscalling issue, this can end up really bad.
What I can do to make my ex stop miscalling me??

Stop answering. That's all you can do. Eventually he will get the hint.