As per the previous post, EE signal boxes use 3G which is being switched off, and their switch-off is part of that process. This is an industry-wide process that is not unique to EE.
The recommended solution for any indoor coverage issue has been WiFi-calling for several years now, what phone do you have? If it's older than a few years, your best solution is likely to be changing to a newer phone that supports 4G-calling & WiFi-calling - both of which bring several advantages over the older 3G technology.
