I am also just a hobbyist so please excuse me if I say anything misleading or incorrect. Not long ago I completed an Arduino project in which I used the Arduino to mimic the serial output of one of those decoder chips. My goal (which I achieved), was to use an old Radio Shack caller ID's LCD like a serial LCD module. My caller ID had two circuit boards with a ribbon cable connecting the two. On one board were two phone jacks, capacitors, diodes, resistors, an oscillator, and a decoder (EM92547BP). It also had two leads that ran to the 4 AAA batteries. The other board had the cpu + LCD controller (epoxy covered), the LCD, buttons, etc. In my project, I bypassed the first board and used the Arduino's tx pin to send fabricated caller ID data to the second board for display on the LCD. In theory, if the Arduino can mimic the serial data that is output from one of those chips, it can also receive data from one without difficulty. It seems that the board I bypassed would be nearly ideal for your project. It accepts battery voltages (so 5 volt from the Arduino should do), it has phone jacks, required circuitry, the decoder chip, and it outputs serial data through the ribbon cable when CID signals are received. One of the ribbon cable leads is pulled low when a ring occurs, which might be useful. There is one downside, the ribbon cable only carried the full serial data output (preamble included). If you wanted the SHORTDATA output you would have to solder directly to the chip.
Hopefully you will be able to find a caller ID module with the same configuration as mine. If you do, I would imagine that interfacing it with an Arduino would be one of the easier stages of the project. If I am correct, you would simply remove the board from the caller ID, connect Arduino +5 volt to bat+, Arduino GND to bat-, and Arduino RX pin to serial data out. Optionally, you could connect an Arduino pin to the ring indicator output to get a heads up on when to expect serial data. If you needed to use the RX pin for something else, you might make use of the software serial library in combination with the ring indication so you would know when to listen.
EDIT: Oops, I just noticed the age of the first posts. Maybe I shouldn't have spent so much time replying, as the project may have been long forgotten / concluded. Oh well.