Just guessing -- it's a doodad that the phone companies used to hang off nearly every circuit, that responds to certain tones back down the line so that the circuit's physical integrity can be verified over a long distance without sending a technician over. Likely handy in e.g. rural USia.
Also likely to fuck over DSL transmissions, if the test signal and response signal sit on the same bands as A/S/G/whateverDSL goes over.
What I don't get is why they epoxied it over, especially half-assed like that. It's pretty clear there's a fuse in there, a rather big diode, a couple of caps, some resistors and a custom IC.