"that is normally used as an external connection box "
Normally as in normally internally before the master in a typical property? Never had this in my previous properties.
the external grey box looks like this and not like yours this is a master socket usually outside the back door
Out of curiosity anyone know what this is. I have a grey BT branded box immediately before my master sockets (2) its oblong but also slightly triangular with a piece of string/cord coming from the front to the bottom.
do you have 2 master sockets? do you mean one actual master and and other NTE5 used as an extension socket?