Listening to long, non-interactive, telephone broadcasts can quickly become annoying. With a regular telephone, you quickly get tired of holding it. If your phone has a speakerphone feature, and you use that, you quickly discover that unless you have a premium phone (one that costs about $300 or more), that the speaker quality isn't very good. If you had planned to have a group listen, and have to sit back from the speakerphone, you generally discover that it isn't very loud either, and that if you raise the volume to the max, you get a lot of distortion. For solo listening, a quality dual-ear headset can solve the problem, and works for two-way conversations also, but the bill for that quickly hits $200 or more.
Business teleconferencing equipment can fill the bill, allowing a group of listeners, but such systems generally start at the price of a premium phone, and go up from there. Besides, a lot of the expense is in supporting two-way conversations -- quality omnidirectional microphones, even extension microphones; and echo suppression circuitry.
So first inventory what you have. Start with speakers. Do you have some speakers attached to a stereo, boombox, a computer with amplified speakers, an iPod dock, a public address (PA) system, or even a guitar practice amplifier, or an organ or electronic keyboard? If it has a microphone input and/or a line input, you can use those speakers, and fill a room with telephone sound. If you have none of the above, a guitar practice amplifier, because it is not stereo, yet usually has a decent size speaker, might be a reasonable investment. They can be had for as little as $60. Beware, though, if the speakers on your device of interest are tiny, they will be tinny, and not signficantly better than the telephone speakerphone.
Think about other possible uses you might have for some speakers; this could influence the type of speakers you buy. Powered speakers for a computer can be found cheap, but don't get the "USB powered" type, unless you plan to use the computer as your amplifier for your telephone conference. Cheap computer speakers often have extremely small, tinny speakers. A practice amp such as this one has a decent size speaker, and is not too expensive.
Now the remaining problem is how to hook up the telephone to the speakers. Check out all your existing telephones. Does one of them have a built-in headset jack? If so, all else you need is wire! The exact right sort of wire, of course, but a quick trip to Radio Shack can solve that. Although for long broadcasts, it would be nice not to depend on batteries, and hence a wired telephone with a headset jack would be preferred, more cordless and cellular telephones have headset jacks than wired ones do. Should you run out and get a wired telephone with a headset jack? Well, you can, but they tend to start around $50, so read on.
If you have no telephone with a headset jack, or it is cordless/cellular and you need to operate it longer than the battery's charge lasts, then look for a telephone recording adapter. In spite of their names, you don't have to record with them! They are made to interface to the line-in or microphone-in jack on various recording devices, but those are the same jacks that you just (hopefully) found hooked to a set of speakers. These devices hook in various ways to various types of telephones. One that will work with nearly any type of telephone is this one, and it will work with nearly any type of telephone, from home analog phones, to business analog or digital phones of most types. But at $50, you might as well buy the wired telephone with a headset jack.
Another device that will fill the bill is a recording adapter by Gooods.com (count the "o"s!). This will only work with analog telephones (home or business), but is _very_ cheap, although with expensive shipping.
How to hook it up, depends on what you are hooking up, of course. Unfortunately, there is no single standard for various audio jacks for various types of devices. The telephone signal is a monaural signal, not a stereo signal. So you need a cable with only two wires, basically. But some of the devices have stereo or multipurpose jacks. Here are the wires you need by type of device.
Warning: even though mono plugs will fit in stereo jacks and stereo plugs will fit in mono jacks, you cannot, in general, convert from mono to stereo by doing so. A stereo jack will ground one side when a mono plug is plugged in, and a mono jack will only pick up half of a stereo signal, perhaps the wrong half.
If you figure out exactly what you need for a particular combination of devices, you can make a single cable with the right type of ends on it, which is simpler if you can make your own. For some combinations of devices, you might find exactly the cable you need, though, so look around before building one. You can nearly always find some combination of cables and adapters at Radio Shack to fit any need.
If it is a PA or practice amp, it might have monaural inputs; otherwise you are most likely looking at stereo inputs. To hear the signal on all speakers, you need a stereo-to-mono adapter or cable. Depending on the size and shape of the plug on your device-with-speakers, you might need 1/4" 274-1520, 1/8" 274-374, or some combo size phone plug adapter or cable, or for separate left/right inputs, a Y-cable or adapter such as THXAI-YFNF or 42-2535.
Most phones have 2.5mm (3/32") TRS headset jacks. These use a stereo plug, but the signal is split between input and output signals. We are interested in the output signal. These are not common, so you might need this adapter 274-397. To split the signal, you'll need a splitter cable 42-2550, or an adapter 274-369. Only one side of the adapter will be used to run to the speaker input (whichever one works, and it won't hurt to try both to figure it out).
These usually come with some wires. Mostly you just need to adapt the cable ends, for your line-in plug. As shown in the other sections, there are lots of adapters available, to compensate for the lack of (or multitude of) standards.
This comes with what appears to be a stereo Y cable, but it is really a mono to double-mono adapter for recording two mono signals via a stereo recorder. So if you use just one device, you can get a stereo-to-mono converter for the end of teir cable, and leave the other end dangle. Don't use a mono plug into their device, though, because you will ground your signal.
A Goods recording adapter, and a Behringer practice amp look like an inexpensive way to get a whole solution, if you have nothing already on hand, and no other needs for the equipment. You will also need two adapters: 274-325 and 274-368. P.S. The practice amp, plus a microphone, can also be used effectively as a small PA system.