fr the sky website:
Your Sky Multiroom subscription covers the address of your home in the UK, Isle of Man and the Channel Islands, where your Digital Satellite Equipment is installed. All the Sky boxes you watch must be connected to the same phoneline capable of making outgoing calls. This enables some Sky services to work, so you must tell us immediately if the line or number changes. Each Sky box must also be connected to a mains electricity supply and have its own viewing card. If you have taken up the free Sky box and minidish offer, the Interactive Discount contract, Sky multiroom subscription(s), Sky+ and/or Sky+HD, then as part of the installation we must connect your box to a working telephone line. If an appropriate socket is not available or the Box cannot be connected to such a socket for any other reason, we will not be able to deliver or install your Digital Satellite Equipment. Instead, we will arrange a later installation date once you tell us that the appropriate telephone socket has been installed. In connecting the Box to the socket, we will use the nearest available socket and any cabling will be clipped to the relevant skirting.
so.. i guess each box has to be able to plug into a telephone socket that has your main telephone number.
If youre unsure.. call them.