Unable to send text messages (including to non apple phones) and unable to activate imessage using phone number

I have an iPhone 8 on Belong (telstra Australia) network that is suddenly unable to send normal text messages to any phones including to non iphones. I have over time experienced temporary issues where I get the red exclamation but usually that corrects itself and I'm able to send texts again.

I can send imessages (using email/apple ID) but cannot activate imessage linked to my phone number I always get an activation error when attempting to activate imessage in this way. I cannot set it to send and recieve from my phone number, only my email.

So far I have done the following:

• Restart Phone activate and deactivating imessage many times.
• Try a different simcard. The new simcard works no problem, it activates imessage linked to the new phone number so I know its not the phone that's the issue
• Updated to latest IOS 14.0.1 from ios 13.5
• Reset Network Settings
• Signed out of all other apple devices
• Attempted to deregister my phone number on apple website to try and reactivate but it says my phone number is NOT registered for imessage...which I agree with, I cannot switch to it in message settings it says activation error.

My internet 3g/4g works fine. I can RECIEVE text messages but cannot reply to them (red exclamation).

I'm out of options. I am considering porting to a new phone provider to see if that would help.