between the wall jack and the modem, no deal,slim, you'd need to decode the carrier, and see what the data does from there
however it may be easier to put one between a router(pc) and a modem, to catch all data going through in its digital form, I looked up the pin diagram and it says there are 2tx and 2rx wires, if thats all there is to it then its just a simple matter of using a counter on each wire, where you can then add up the tx and rx bits,
of course i could be over simplifying, and you counter needs to be fast enough to catch the bits, also you may not be accurate since that wont catch all the 0's unless you advance the counter to read by bytes,
just a guess thou, may want to wait for a pro opinion.