We have a device in the house that complains if it doesn’t have a connection to a phone. It detects this by checking for voltage coming from the line, not by listening for a dial tone (landlines have their own power—see here).
We don’t want to connect it to a phone line. Is there a ready-made device (or a relatively simple one I can put together with a little help from Radio Shack) that I can plug in that will mimic the phone line’s voltage output?
If all it is checking for is voltage and it never tries to dial or or check for a dial tone, then all you need are some batteries and a resistor. Phone line voltage is 48 volts DC. Get yourself four 12 volt batteries and hook them in series, and put a resistor in series with those to limit the current. The resistor value isn’t critical but you’ll want it to to be above 1k.
Wow, sounds fairly simple, thanks (I assume it won’t take much to figure out/Google resistor values and whatnot).
Aren’t 12 volt batteries somewhat large? Is there a power adapter that I could plug in that will negate the need for them?
ETA: Or what about something that would plug into a USB port? (ETA2: nevermind. It looks like USB ports only pout out 5 volts)
I’d bet that it considers anything over 6v or so a valid line…
You could use six 9V batteries - handy since you can plug them in a series string. Plug the positive terminal of one to the negative terminal of the next one, to form a line of them. You might get away with fewer (maybe even as few as one or two), depending on the threshold in the detector. Even though the nominal voltage is 48V, it can be much lower or higher in practice, so it’s likely the detector would be OK with something considerably less than 48V.
Don’t forget the series resistor.
12 volt batteries can be huge things that start diesel truck engines, or they can be tiny little things that go in smoke detectors. The bigger ones can supply more current, which you definitely do not need in this application.
Five 9 volt batteries in series would be close enough. Heck, four would probably do it. Phone line voltages vary a bit due to the long length of wires involved. I’m sure your device will trigger well below 48 volts.
Your typical “wall wart” power supplies don’t tend to be that high in voltage. 48 volts is used a lot in industrial use though, so you might be able to find something like a 48 volt din rail mounted power supply fairly cheap. You don’t need much current, so something small and cheap will easily do the trick. With something like this you could just plug it in and leave it. You wouldn’t need to replace batteries.
USB is only 5 volts. Maxim makes some integrated circuits that you could use to bump up the voltage to somewhere in the range of 48 volts without too many external components required, but this is getting a bit more complicated. What’s your electronics skill level?
Not necessarily As an added bonus, you can take it apart and get eight 1.5V button cells.
It’s pretty likely you won’t need nearly 48V - 10V will almost certainly be enough, maybe even lower, as beowulff suggested. Office PBXs are often 25V or less, for example. I’d try a 9V battery, then two in series first.
Remember those Radio Shack toy boards with springy clips, built-in resistors, brightly colored wires, meters, etc. and a book of things to do? Man I loved those things. But that was about thirty years ago, so my knowledge may be a bit rusty. That is, I just Googled “series resistor” and “48 volt din rail” to see what I’m looking at.
Battery-less would be kind of important, since laziness abounds.
Your series resistor is just a simple 1k or larger resistor from Radio Shack. Looking online, it looks like you can get them in a 5 pack for a buck.
I’ll see if I can find you a cheap 48 volt plug in power supply of some sort. I’m off to google.
Take an old wall-wart adapter rated at 12V DC or higher (the older the better - older ones are likely to be much higher than 12V with no load - can be 20V or more) and cut off the plug. Take an old phone cord and cut off one end. Connect the 12V adapter wires to tip and ring of the phone cord (usually red and green) - you can get a way with just stripping them, twisting them together, and taping them, at least “temporarily”. That’s pretty likely to work. You could experiment with batteries first to see what voltage you actually needed, but I’d probably go for broke with a wall adapter first.
Note - obviously use caution, no warranty implied, etc.
Just so you know what I’m thinking of, here’s a 48 volt din rail power supply.
http://www.trcelectronics.com/Meanwell/mdr-40-48.shtml
At 32 bucks it’s a bit pricey for what you’re doing, but it would work.
Dish Network receiver?
If it’s an alarm system, look for an option to disable the line monitoring in the programming. Way easier than faking a line.
It’s an alarm system. Unfortunately (for exceedingly boring and non-germane reasons), accessing its programming isn’t feasible.
So $32 bucks to plug it into a wall or $5 in batteries? That’s quite a disparity. What makes the difference?
Skype USB adapters emulate phone lines. They can ring phones, use phones as a headset for skype, etc. $9.99 with rebate. You’ll need USB power or a computer nearby.
One’s a home-made cobbled-together solution and one’s a world-standard manufactured device for industrial use with a three year warranty. 
Speaking of warranty - obviously there’s no warranty on a pile of batteries, but as the power draw is essentially none, the batteries should last for whatever their advertised shelf life is.