Blacklisted S7 has signal but no mic

I recently send my S7 (which I got on ebay as "brand new") to have the LCD repaired. I had it for four months and it was working fine before I dropped it. I was told from the repair place that they had issues with the mic; it wasn't working so when they checked the IMEI, it came back as blacklisted. They couldn't get any signal and the way they explained it to me was that it was useless as a call making/receiving phone and could only use it on wifi.

I received the phone yesterday and I put my SIM card. Not only did I have reception, I had a working 4G signal, could access the internet and was able to receive calls from my colleagues. Only problem? They couldn't hear me and I could only hear them on speakerphone, not through the external speaker at the top.

At home, I tried the Skype bot that lets you set up the mic and all and that was working, again only when I had it on speakerphone. Same on Google, when I say Ok Google the assistant comes up and I can 'voice search' just fine.

So I'm confused. I thought blacklisted means you get no signal, you can't make any calls etc but can a blacklist status affect the speaker/mic when on calls? The repair was about replacing the cracked LCD, could the new screen affect/disable the mic since the phone is not a European model (I got it from a now banned from ebay seller from Hong Kong)? And finally, should I be trying to un-blacklist the phone or fix the mic and the speaker?

Any ideas/recommendations are more than welcome. Many thanks in advance.