Sorry to go so long without responding, but I had to wait for the malfunction to return!
This evening, just on a wild hunch, I checked to see if it was back, and got lucky.
So I called myself from my fax/modem line (after making sure nobody else picked up the call), with the following results:
Ringing stopped when I picked it up (which was as before, actually).
I could transmit (at least this time), but could not hear.
I then swapped phones between my main line and my fax/modem line. Definitely the phone, not the line.
Violent shaking of the base had no effect.
Unscrewing the mic housing on the handset got me dialtone, the very moment the cartridge came loose (yet I'm sure I successfully talked to the other line -- how could the mic be pulling it down and still transmit successfully?)
