May 25th The Mystery Box

If anyone can explain the following to me, it would be greatly appreciated. We discovered this box attached to the side our house, with a cable from it going inside to under the house.  We checked with the neighbours and they don't have anything similar. I have had phone problems lately, but this doesn't appear to be anywhere near our phone line source. That is an actual slot on the front of the box.

Can someone shed some light on this please? Or is Big Brother watching us - of all people???