Oukitel C13 Pro SIM card not registering number - Not Making or Receiving Calls

I have a SIM that works in any other phone, carrier Aldi/Telstra.

When the SIM is in the Oukitel, it won't make or receive calls.

The phone is not registering the mobile number of the SIM, so when I go to settings, I get a SIM card error message.

A telstra SIM was put in the phone, and it worked fine. It registered the number with the phone, and showed the number in the settings.

With the Aldi SIM (brand new), it says number unknown in the settings.

The SIM works as it is using data - Facebook, email etc.

The seller insists that it is a SIM problem, but both Aldi SIMs that I use work in any other phone.

Can someone help me please? I can't get help in Australia.

Why wouldn't the phone be able to detect a number on a SIM for a certain company?

Thanks in advance!