Elderly people living on their own often have an emergency call system which is a button they can press that calls an ambulance service. For that to work, all phones in their house need to be on the hook. Naturally they have phones all around the house upstairs and downstairs, and it's easy to leave one off the hook. So what is needed is an off-the-hook indicator, which is a very simple circuit with a red LED that shows when a phone is off the hook. No other features are needed.

I have seen some such devices for sale but are out of stock, or they don't work in some countries or with broadband. So I could make one, but I am having trouble finding the specified transistors, e.g. BS108. I am told that nobody bothers making their own circuits these day so nobody bothers stocking transistors. My favourite shop says that when they have sold the transistors they have in stock, they will get no more.

So what I want is a circuit that lights an LED at about 10 volts DC and turns it off at 30 to 40 volts, and with components that I can actually buy.

I'm not interested in discussions about how people should hang up their phones, or any other solutions.

This looks a suitable circuit (but I can't get the transistors):
http://www.zen22142.zen.co.uk/Circuits/Misc/tiuc.htm