How many branches are you using? What type of line splitters are you attaching the extensions to? When you say you're lifting the phone, which makes both upstairs and downstairs ring, is this when you are answering a call, or even when you are about to make a call?

The phones will ring when a signal is sent and the circuit is closed (ie - the reciever is down). The phone shouldn't ring if you have the reciever in you hand. It could be a faulty signal splitter, but I'd need more info.

see, thats what i said...i just made less sense

The phoe rings when we pick it up to make a call. Also found once that for an incoming call that phone didnt ring, but it usually does.

Our 'branches'

Main line comes in and is branched at the mainbox. the downstairs has a standard 'divider' socket for two phones.

Upstairs the box that lead goes into splits to 4, a skybox, a phone, my laptop, and my pc, using similar splitter sockets. However, taking all that out and just using the extension and the phone upstairs still creates the problem.

Once in a while downstairs weve had a phone fail to pick up a call, pick up the receiver, and the phones continue to ring all over the house. That phone is a cordless tho, so we figured it was a problem with that phone.

I fear that if we call in a BT engineer the first thing he'll say is 'you cant have all these appliances on one line' and make us get a second line, but this setup has been fine four a couple of years...

Thanks for you input

OK, first off, yes, having too many devices on the same line can cause problems. Basically, there is a set voltage being piped down that line which the phone uses for it's dial and ring tone. Place too many devices on the same line and you'll devide the current too much.

Now, let's see if I understand you. You have two splitter points - 1 at the BT point, and a second one upstairs, splitting into two and four respectively. However, if you have only one device on the line after the initial split then the fault still occurs. This sounds to me like the source of your problem. When you are lifting the receiver, the signal should be transmitted straight out of the house, but it appears that at least some of the signal is being rerouted to the second line, thus causing the phone to ring. The fact that sometimes the downstairs phone doesn't ring when there is a call from outside the premises adds to the impression that the initial splitter ain't doing it's job properly.

You'll either need to replace it or get an engineer in. Replacing it might be cheaper in the short term, but without testing the current at the various points in your set-up there is no guarantee that this is a long term solution - which is what an engineer should be able to offer.

Good luck!

Well I have to say it sounds like a short on the bell wire on the cable to the upstairs phone, either that or a new device with a particularly high REN number has been plugged in.
I would try plugging all the phones you are using one at a time into the main socket, and ringing them to ensure none of them are faulty, and then I would suggest unplugging any extension lead/splitters plugged in upstairs and just try ringing a with one phone upstairs and one downstairs, then try disconnecting the cable from the downstairs phone point and plug a couple of phones in and ring up.
It's a process of elimination - but a piece of gripper rod/ or a cable cleat through the cable could cause these symptoms.
Hope you get it sorted without calling out the dreaded BT!!

Anyone know what REN number a Sky+ box is, also do devices that are connected, but totally powered down, like a PC with its modem connection still count?

Sounds very much to me like the wiring from the Main box to one of the extensions is reversed hence the ringing on one of the phones whilst the second is being used.
Are the extensions plugged in via doublers or hard wired into the sockets?
The good news is that I have never hard a phone system that I have not been able to discover that fault.