How am I contacting via text message but if I call the number it’s disconnected

One of my children’s mobile phones needed a new SIM card when activated I had previous mobile number connected not the sim card number it would come with . Here’s how it gets strange.

so if I wanted to call that family member I called them on the number added on activation when I texted that phone number it didn’t send as an iMessage. It send it as an SMS , I have that if unable to deliver send as a Sms turned on , so the family member still received my text messages yet if they called me it was from a completely different number yet if called it it was a disconnected phone number . ( yes I know sounds weird )

But the moment I rang up our mobile service and ask them why there were two separate mobile phone numbers connected to the SIM card. They were surprised and confused because it only showed the number I activated once doing that. The other phone number completely disappeared. Now I don’t want to know why cause I’m pretty sure it’s probably a third-party messaging app but how do I access those details of that Random phone number that was connected to my child’s device because I’m sick of getting the reverse my number though whose number is this find my IP address? No I don’t want that government ****. I want somebody to tell me exactly how I can find out who’s that phone number was. Because also the craziest part was her phone started glitching a brand-new phone started glitching and she wasn’t able to access the screen on multiple occasions. She would press it wouldn’t turn on and then she would leave it and then come on and I know the motherboard is underneath the phone but come on. We’ve got the whole motherboard. Kunt have been on the fritz so the whole screen doesn’t work. There should be a part of the screen that worked..

But I don’t care about that. I want to know how I can find out who’s and where that number came from and how were they able to be linked to my childs phone?