The plate in picture one is an after market master point, as the yellow "bell ringing capacitor" confirms.
The two wires connected to it would normally lead you to believe it to be the master point.
The other picture doesn't show the whole of the plate, but the three wire system would normally denote an extension socket. Have you any other phone sockets in the house? If not then this could well be feeding the master socket in pic one, even though it shouldn't really be.
The box outside is indeed a junction box. The sockets have probably been added from there by sky, builders or DIYers in the past.
I used to be with sky, and because I did not have the correct BT master point (which you don't) they sent a BT engineer out to fit one at no cost to myself. You should confirm that with your service provider, as I don't know how much or if BT would charge if you ask them to investigate.
Knowing how it works is completely different to understanding how it works.