You mention 'handsets' several times and that you've changed some of them. What about the telephone sets themselves to see if the problem follows the phone? I know it's a stretch, but worth a shot if you have the equipment at your disposal.

Still, I'm not buying the fact that your 30+ year old equipment just stopped working properly. Keep in mind that this system was designed and sold back when REAL phone companies were the only option for dial tone and they provided REAL specifications on those lines. I'd get a volt meter and measure the voltage of each line, both on-hook and off-hook. You should measure something in the 48 volts DC range on idle lines and around 12 when off-hook. Anything lower than these and you'll have issues (especially the off-hook voltage). There are line boosters/conditioners available that will help to compensate for errant voltages and you might want to consider purchasing some. Before that, especially if your voltages are out of range, I'd make Spectrum tighten up. If they want to sell phone lines, then they need to be real phone lines. Their modems (EMTAs) made by Arris are fairly reliable, but the newer ones made by Ubee have a mind of their own.

Ed Vaughn, MBSWWYPBX