If you are trying out T-mobile’s HotSpot@Home VOIP home phone service, there is a setting you may need to change. T-mobile is still figuring this out so their technicians will invariably blame it on your ISP to get you to hang up and go away. I usually like T-mobile, but they have had this VOIP service out long enough to have figured out these types of issues. By the way, this article is only useful if they gave you the Linksys WRTU54G-TM router (this is the model they are currently handing out with this service).
I have Comcast as my ISP, and I was having the same problem others were having (no blue light — no phone service). I also got the same error message as others (W006.1 ISP Error. Retry.) I have found that the default firewall settings on the WRTU54G-TM router are too strict. I am not an expert so there may be a less drastic fix, but turning off firewall rules solves the problem. To do this, log in to your router via a computer connected to it (type http://192.168.0.1 in your browser to access it). As has been mentioned before, the default user name and password for the router are both “admin” (without the quotes). You can change this later. Once you have logged in, click on “Security” and then “Firewall.” Change “Enable” to “Disable” and click “Save.” You may need to restart your router (just unplug it, wait a few seconds, and plug it back in). Once it has time to start up the error message should be gone now and the blue light that indicates a phone connection should be back on. (To check, log back into your router and click “Status” –> “Voice” –> “Error List.”) You should be in business!
Here is a link to the Linksys forums where I posted this as well:
If you want to check for firmware updates, here is the place (although currently there is only Version 1).