After living in the same place in New York City for years, I recently
> moved to another place (also in New York City). The room I moved to
> has an existing phone jack but it looks different from the old
> square-ish ones I'm accustomed to. (The old ones consisted only of 4
> term> Network Interface
> *Caution
> Disconnect plug from this jack during installation and repair > of wiring.
> *Testing
> Plug working phone directly into this jack. If phone operates,
> fault is in wiring. If phone does not operate, call repair
> service.
> When I opened the case, I noticed that the red and green wires (the
> only ones that will be actually used by the telephone itself) are also
> connected to a little circuit board whose most conspicuous component
> is a yellow cylinder-shaped object (about 3/4" long and about 3/8"
> diameter) with the following markings ...
> 250V
> TI
> 0.47 MFD
> +/- 10%
> * What is the purpose of this circuit board?
> * Is it really necessary? (How come the old-fashioned jacks
> didn't have this?)
> * What if I were to disconnect it?
> Also, when I looked inside the jack itself (the hole where you would
> plug the phone into), I noticed there's some strange-looking gunk
> inside. It's clear-colored and has the consistency of rubber cement.
> * Is this something that's supposed to be there?
> * What is it used for?
The capacitor is probably part of an RF shield on the jack. The goo as an anti-corrosive gel. Copper and brass and all sorts of metals corrode over time. This prevents the jack from getting gunked up.