apms,
If I had to make a guess I would say that you went from an AT&T CO Line with DSL to a Cable line converted to analog via a modem through Comcast. As long as the lines were connected to the same location it should have worked fine, but from my experience with Charter, similar to your Comcast, the level of knowledge and experience of their technicians can be somewhat lacking on how to connect them up properly; not that it is difficult.
Anyway, it is usually best to always have your vendor available to work with a new service provider to alleviate the issues you already went through.
Sorry,
TE