I have been using a Magic Jack with my Comcast service from the start. In October of this year, it stopped working. MJ support wanted me to open certain ports in my router which I did. Still it does not work. I replaced the MJ unit since then and was still unable to use it. When I took the MJ unit to San Diego to my relations house, it worked without doing any special configuration on their router. I also tried it on my next door neighbour's Comcast/Xfinity Modem and it does not work. All this points to something with Comcast but I have not been able to get a satisfactory answer so far.