It likely refers to the technical definition of the core of a telco network, see e.g. the evolved packet core part of a 4g/lte network https://i.ytimg.com/vi/6d

It likely refers to the technical definition of the core of a telco network, see e.g. the evolved packet core part of a 4g/lte network https://i.ytimg.com/vi/6dt9xVMvtB8/maxresdefault.jpg https://i.ytimg.com/vi/6dt9xVMvtB8/maxresdefault.jpg , of which the eNodeb's ("phone masts") are not part of. At least for 4g some layers of the control plane and all the user plane data are encrypted when passed through the eNodeb's. Albeit an eNodeb can probably do a lot of nefarious things if it wants to. Or they mean literally just the phone masts and antennas - which are just inert components.