Wifi calling

Hi all. I have a new Telstra SIM card in my iphone8 and it is working for all services except wifi calling. This is critical to us as we have no mobile reception where we live. I have tried turning it on and off, have removed and reinserted the sim card, have reset my network settings, the carrier settings have been updated & I am using ios 13.6. Alas still no joy. Does anyone have any suggestions?