Is your own phone tapped?
First of all, some people will tell you that there's a phone number you can call that will tell you if your phone is tapped. People email these numbers to me all the time. They say that you call it and you'll hear a contant tone. If the tone does a certain thing, your phone is tapped and if it does something else, your phone isn't tapped. This is stupid. Those tones are called "sweep tones" and they're used by phone company technicians. They have nothing to do with tapping anyone's phone. If someone tries to tell you that they have a number that detects taps, hit them. Hard.
The best way to detect a physical tap on your own line is to carefully inspect every inch of phone wire in your home. Start from the outside, where the phone line comes from the pole. Open up your grey TNI box and look for anything unusual inside. You can also open up the "restricted" side of the box and check in there. It requires a special alan wrech that you can probably find at your hardware store. After inspecting the outside, go back inside and figure out where the lines are coming from that box into the house. If you have a basement, it's probably there. Follow every wire, noting anything suspicious. Then look closely at every telephone in your house and trace the wiring back into the wall. Obviously some wires are going to be run through walls and it's up to you if you want to start busting into the walls.
So you didn't find anything? Well there's still a few things to consider. What if the person who's tapped your phone has actually hidden the tap inside one of the phones? Some of the more advanced taps can be very small and easily fit inside a phone. It's not very hard to use a screwdriver and take apart your phones. If you're not comfortable taking things apart, chances are that you know somebody who is. Be sure to take apart both the base of the phone and the handset.
If you live in an apartment building, your neighbors could have access to your phone lines. If they know where the phone jacks on your side are located, they could punch through their own walls and hook up an extension jack in their own apartment. From this extension they could make free long distance calls or they could put a tap on your line and listen to all of your calls. Also, in most apartment buildings all of the phone lines come into one central place from the telephone poles. From there, the lines are run into each apartment unit. At that central location, a tenant could simply add a few wires running from his line to yours and they'd have access to your line. Most phone cables have four wires inside of them - red, green, yellow and black. If you have one phone line, you're probably just using the red and green. The yellow and black wires don't do anything. A neighbor in your apartment could hook YOUR red and green wires to HIS yellow and black wires which would create a second line in his apartment. Which would be YOUR line. If you think this is happening, you could ask the phone company to come and check your lines. You could even claim that you've picked up your phone before and somebody has been on it so they would know what they're looking for.
One last scenario - you know those green rectangular phone company boxes you see out in the streets and in back yards? The ones by your house contain your phone lines and probably 100 or so other phone lines of neighbors. People have been known to open up these boxes and hook their own phone into them. It's easier than you would think for a person to figure out which of the lines in these boxes are yours. They could even do the yellow-black wires red-green wires trick to create an extension of your phone in their house. Even if their house is several blocks away!
With each phone device you hook to a phone line, you draw a little more voltage from that phone line. Radio Shack used to sell "phone tap detectors" that would detect these extra draws in voltage. Apparently it could tell the difference between one phone being picked up and two phones being picked up so you would know if someone is listening in on an extension. I think this would also work in detecting a physical tap on your line. Radio Shack doesn't sell this item anymore, but you can buy a Tele Safe II Telephone Tap Detector (pictured at the left) which is pretty much the same thing, albeit just a little more expensive.
Finally, there are government taps. If you're a bad person and the government thinks you need a tap on your line, they can do it and you won't be able to detect it. It's done from the phone company's switch and there's nothing you can do to prevent it. They can tap your cell phone calls the same way. Your only way to escape government phone taps is to go use a pay phone or a prepaid cellular phone. And stop doing illegal things that makes the government tap your phone, you damn criminal.
How to record phone calls using voice modem?
You can use any voice modem connected to PC in order to record all phone calls in your home. Below are some links to a well-designed software products: