Deregister iMessage on iPhone doesn’t work, deregistering iMessage online is not possible - missing country dial code!

I moved from iPhone to Samsung and I still have my iPhone with me. I turned off iMessage and FaceTime with my sim card in my iPhone and registered to my home cellular network.

Messages settings screenshot:

FaceTime:

Now I tried to send a regular sms from my iPhone to my wife’s iPhone and I asked her to send me some reply for every SMS. She received all my green bubble sms-es and she replied on each of them, but I never received any sms from her on my iPhone:

But then I checked my Macbook and all of her replies were there:

Screen shot of my iMessages still showing on my Macbook and never received as SMS on an iPhone:

Why Apple keeps my phone number registered for iMessage and FaceTime? I was on my cellular network when I switched off iMessage and FaceTime!

I can't reach Apple on their support phone line! I am not receiving any messages from iPhone users on my Android phone! This is extremely frustrating! Then I also tried to make a deregistration online, but in their online form they don’t have my country Slovenia on their list at all, so I can't deregister my phone number online:

My clients send me text messages that I never receive and I am loosing their confidence by not replying on time, since I am their software developer on contract. I never thought Apple is so hard on people switching from iMessage to regular sms messages.

Can anyone suggest any solution? Thank you.