received an sms from my stalker after changing number If anyone is experienced with iphone imessage and sms, please enlighten me.

There is someone stalking me by keep calling and texting even though I told him not to. It's been going for too long. The following things are the sequence of some key events in this:


Both of me and him own iPhones
I changed my number (same provider but different number, but did not change my apple ID). I also put my new number in the "my number" entry in the settings and did the same things to all my apps and blocked his number.
Four Months later, I receive a message from an unknown number and I asked who it was and it turns out it is the stalker to my surprise
his number in the form +61-4xxx-xxxx which is a standard Australian mobile number was showing at the top in his message (like the attached image although please note that the thread if not the actual message from him but it is a similar example as the text bubbles were blue and gray were showing in the thread from him yesterday)
I never put my new number on FB or anywhere
When I asked him how he found out my number, he just said "I just replied to the same conversation in our conv" which I think is a lie
Yesterday, I changed my number again, and this time even my apple Id and blocked his new number. Reflected the new number in all apps
He works for a major telco company and has about 10 smart devices and probably unlimited number of sim cards. This is not the first time he tried contacting me from different numbers. Because his work involves testing phones, he is quite knowledgeable with iphones


My question is: how he could've texted me although I got a completely new number? Is he telling the truth? Or do you guys think he somehow got a hold of my new number from someone? Is it because I did not change my apple Id and he was indeed able to get to my by getting a new number and imessaged me? I do not understand the imessage versus sms business. Please help me, so I know how to prevent him from getting my new number.

Thanks.
If the message is in a green bubble, it came as sms, if it's in blue, is came through Apple/iMessage. If you go into the Messages settings on your phone, you will see Send &amp; Receive addresses and you should see how his message arrived. It will probably your apple id email there and he won't need your new phone number.