Have you got T38 fax enabled in the linksys? If yes then try it with that off, if no then try and enable it. The problem you have is that, as i see it, you have multiple servers in the line and I thiunk they are fighting each other. I do wonder if the Linksys needs to be set up to the MBG rather than the 3300 to give you in effect a separate analog line out rather than an extension on the 3300?
just a shot in the dark really.