I have 4 regular POTS lines in my home. I use 2 for voice and the other 2 are used for 2 separate PCs that have dial-up access to my company's VPN. On too many occasions to be co-incidental, when one of the lines I use for voice calls rings, one of the two that is connected to the intranet will get disconnected. All 4 lines are in the same cable from the street to the demarc at my house, and from there, the 2 pc lines are in a separate cable from the other 2 lines. It's hard for me to believe that there can be an interaction between these, just 48v and low freq., etc. Anyone ever heard of such a thing and if so, how to remedy the situation.
make a voice call from one of your voice lines to each of the PC lines. They should complete normally. Repeat calling from the other voice line.
repeat the above, only originate calls from the PC lines and call your voice lines. Calls should complete normally.
Make a voice call between the "voice" lines and originate voice calls from your PC lines. Calls should receive busy treatment or call waiting tone.
Make a voice call between the PC lines and originate voice calls from your voice lines to the PC lines. Calls should receive busy treatment. If you hear CW tone (this could cause disconnect), make sure you disable call waiting in your Network and Dialup connection for your PC lines.
With your PC lines both dialed up to the internet, use your voice lines to call each other (2 calls). See which one causes the PC line to drop. Record and call your telco...
I know this doesn't answer your question as to why this is happening, but at least you can determine if everything works voice wise.
It's fairly common. When the incoming call rings the phone, it does so with "ring current", which is superimposed on the 48 VDC loop voltage. The "ring current" superimposed onto the loop current results in peak voltages as high as perhaps 220V.
Compare that 220V to the peak levels of a modem's received signal, which are down around .25 VAC. When one of those phones rings, the cable has a signal that is almost 1000 times as much voltage as the modem's desired signal.
What normally prevents the two signals from interfering with each other is the use of a "balanced circuit" on the telephone cable, but the ability of that configuration to prevent interference depends on the cable pairs being *very* well balanced. Any imbalance, and other signals get mixed into your telephone connection. There are three "signals" that are usually strong enough to be detected first, when a cable become unbalanced for whatever reason. One is ringing current! (The others are 60 Hz from power lines and the clicks from the 48 VDC loop voltage anytime a telephone goes on/off hook or uses pulse dialing.)
Other indications of an unbalanced line are actually hearing signals from the other lines! For example, the voice caller being able to actually hear the modem tones on the other line. You can imagine that if it is bad enough to hear the .25 VAC signal, that 220 VAC signal is *really* going to be loud. Loud enough to totally confuse a modem for example, and cause it to hang up.
I don't see any value in any of the above tests.
The test is valid, but calling the telco might not be the right idea.
The most likely causes are damaged house wiring, or use of the wrong type of cable for house wiring. Anything that causes the modem lines to be unbalanced will cause them to pick up "crosstalk" from the other lines. Examples would be defective telephone sets, corrosion on terminal, staples through the cable, broken insulation allowing contact with other wires or objects, kinks in the wire, and/or being damp.
However, one very common cause is the use of cable that is not a twisted pair. A great deal of "telephone cable" meant for wiring a single phone line has 4 wires (red, green, yellow and black) and is not twisted pair. Using that cable to run two phone lines is a common cause of the OP's exact problem.
Hence, rather than call the telco (unless one has a contract with them to maintain the house wiring), it is a good idea to throughly check out the house wiring first.
The best way to check the house wiring is to determine exactly which modem lines are disconnected by calls on voice lines, run a totally separate cable to that modem and disconnect
*everything* for all lines at the telco demarc, and then place a voice call and see if the modem is still affected. With everything else disconnected and a proper twisted pair cable to the modem, it it disconnects when another line rings there is little question that the telco has a problem. If it doesn't disconnect... finding where the problem is in the house wiring is just a matter of connecting and disconnecting various pieces until it is narrowed down to something that can be replaced or repaired.
I'll just chime in and join the quest for balanced lines.
The design of the E1 and T1 lines has had a lot of scrutiny to assure that they have an EXACT equal time of + and - differential on the same wire, and that the resultant frequencies occupy a pretty narrow band in "analog space".
The major reason is to have zero corrosion and have good properties on balanced circuits.
These are the "six signal killers" (corroded, wet/damp, bent/cinched, insulation problems, impedance/gauge/cable differentials, and bad termination.). You can measure them individually.
Corroded wires give effects that fine-tuned transistor/diode testers pick up.
Wet and badly terminated wires show up on capacitance. bent, Impedance/gauge and termination show op on inductance. This includes non-twisted lines. Insulation problems show up on current leak tests.
All of this can be done with a good line tester; like a specialized multimeter and a few termination blocks and signal generators for the other end. It used to be part of a lineman's set; but now they send specialized crews with fluke or HP equipment that does all the tests in one go and writes a certification sticker.
I also give a piece of advice to all new installs involving DSL or ISDN : do the ISDN/DSL install as close to the demarc as you possibly can without compromising the equipment. Then run your own wiring as S or T ISDN, or Etherent for DSL inside the house.