Why does my iPhone say "Phone not allowed"?

Hello,

I recently bought a second hand iPhone 6, and went through the steps to restore it so I could set it up.

And it was all good except I could never connect to my network, and it always said "SOS only" top left.

So, I tried going into: Settings > Service Provider > Automatic
And switched it off, to then choose Optus.

I restarted, and it displayed Optus in top left, but almost immediately went back to "SOS only".

So I digged deeper into settings, and went to: Settings > General > About > Network
Because under "Network" it said "Not Available", but when I clicked on it, it said "Phone not allowed."

I have a feeling this is related to why my SIM isn't working.

Can someone please help explain what this means fully, as well as help me figure out a way to connect to Optus for more than just a second upon rebooting?

Thanks!