Well, I installed a whole-house splitter, thinking that my problem was with the dongle-type splitter that my ISP provided with DSL modem; but using the phone on the same line that the modem is on still terminates the DSL connection.
Details: Phone service is Verizon, via UTP wiring installed to my apartment within the last two years. Wiring on the Telco side in the NID is sort of haphazard (wires not twisted right up to the screw-terminals), but I'm reluctant to mess with them. Screw terminals have red and green wires to the gray module in the NID that has the user-side screw terminals (also red and green).
I've used a Cat5 cable to connect each of the two phone lines in the NID to a 2" x 3" electrical outlet box just below it, containing (only) Leviton snap-in RJ-12 jacks; done carefully, with spade connectors on the NID side, punchdown on the jacks.
Cat5 cable with RJ-12 plugs connecting the DSL line from jack to a Wilcom PS-15 whole-house splitter.
Cat5 cable, again with RJ-12 plugs from splitter to DSL modem. Linksys WRT54G wireless-capable router (wireless disabled) connected to DSL modem, and serving our home network (again, all Cat5).
All phone wiring downstream of splitter is Cat5, installed by me, and star topology, though that doesn't matter, since I get the same results if I connect our phone direct to Wilcom splitter. I can get dial tone, and dial, without losing the DSL connection, but as soon as ringing begins on the line being called, the connection drops. DSL light on modem relights when phone use is complete, but it takes awhile for the router to
The phones we're using are 2.4 GHz wireless models, if that makes a difference.
Can anyone hazard a guess as to what might be causing this, despite my best and careful efforts? I was vigilant to keep to the USOC and T568A color codes while making up connecting cables, but I might have swapped a tip/ring pair in the phone cables; could that cause this annoying problem?