They might be able to tell from the telephone exchange which IP address is assigned to. However, I'm betting their estimation is based on a combination of factors, such as browsing habits, and mobile devices (with GPS and other geolocation services) connected to your WiFi network, which ultimately communicates with the internet via your external IP address.
To minimise such tracking, use privacy plugins with your browser, such as uMatrix, uBlock Origin, Self Destructing Cookies, HTTPS Everywhere. For your network, use a VPN, with an exit point somewhere else on this planet. As for mobile devices, disable location services for all apps and restrict network access to them as well. White list only the apps you use regularly. You might want to delete google applications, if you are very concerned.