The cable guy (Time Warner Cable, in NY) said this was junk. Not sure if I believe him, but when I removed the splitter from upstream of my cable modem, all my connectivity problems went away.
If you buy a cable modem (to eliminate the $8/mnth lease fee), then don't use a cable that has been split (plug your cable straight from the wall into the internet modem ). Move the phone modem to another room. I was able to split one cable for the phone and TV cables without causing problems.
Didn't work for Oceanic Digital service in Honolulu. Some channels were blocked (including my favorite). The technician came and used his part then it worked well. NExt time, I will know better than to do on my own.