The old style phone line protectors are sometimes referred to as "carbon buttons" which look like an open circuit to the lower voltages on a phone line. When a voltage on the line exceeds a "break down threshold" the carbon button becomes conductive and shorts the line to the ground post after which the button must be replaced.
Newer protectors use combinations of gas discharge tubes or solid state thyristors and fuses or heat coils. These devices provide the short circuit much quicker to short out the over voltage to protect the equipment connected to the line. And like the older device must be replaced if the fuse or heat coil opens the circuit.
MRAM you have provided a real service explaining the phone line. I have stacks of technical data, been to countless tech websites, but you've added detail which was not otherwise rising to the surface.
You have made it sound like this phone line is probably normal. And, indeed, the bell would ring steadily if the 100 volts was more than a phantom. Also, there is no hum audible on the phone, which would surely be the case with real AC.
There's still a problem with DSL being unstable almost always during weekdays and daytime, and accompanying crackle on the phone which rises and falls, and although there is a strong RF component from an AM station at 850 KHz, there is no trouble on weekends, therefore that must not be the real problem either.
This first part may seem "bloggish" but I ended up on this thread after searching "lightning," which in fact is referenced somewhere in here, but the starting topic was "Power Supply." Fortunately it's all worked out because the information pulled from this fishing hole has been valuable, which I've indicated.
There is new information to report. Today's testing of the errant telephone line found that a Sprint telephone answering machine was putting a buzz on the line creating an unbalanced situation. Reversing the wall-wart power supply in the socket did not change the buzz. But to dramatize the effect taking place, when I disconnected the Sprint device there was an instant, loud, abrupt dial-tone followed by a frequency sweep from high to low and then a perfectly quiet phone line! Ever since that discovery the DSL has been running steady and constant without the usual resets and outages.
I give full credit to part15.us and its members for this victory!!!!!!
Great news!
Could be that the phone company system auto-diagnoses line problems. When you removed the problem equipment their equipment responded by re-testing and possibly adjusting equalization of the line. That would explain the frequency sweep you heard.
I had a one piece phone I bought off the "Managers Special" table. Great price but never worked quite right. One day a phone truck showed up and the guy said their equipment indicated trouble on the line, even though our phones were working. When they unplugged that phone their equipment indicated the trouble was cleared.
Threw it away.
Interesting thing about DSL, as an RF carrier on the phone line our DSL never quit working even though we lost dialtone due to an open "tip".