@debsC: Please can you confirm the model name and current Android version of your phone, located in Settings > About phone? Is the issue occurring in all indoor locations, or just certain ones? If it's the latter, have you tried getting in touch with your network to enquire about any issues with the service in that area? In the meantime, if you have a case or cover on the phone, please remove it temporarily in the event that it is obstructing the phone's antenna.
@debsC: I'm sorry to hear that. It does seem as though your home address is a network blind spot, if the device is has full network coverage in all other locations. You mentioned that you had Wi-Fi Calling set up - have you been to the Phone app > Tap the 3 dots in the top right > Settings > Wi-Fi Calling > On, to enable the feature? If so, this should work if you're connected to stable Wi-Fi router at home, as it utilises Wi-Fi as opposed to your network to connect calls. If the issue persists, get back in touch with Three and ask about Home Signal, as this is a service they offer that boosts your signal if you're having trouble making calls while indoors. I wish you all the best.