Tom, I am sorry, but the advice was to get a router, easiest fix in the book. They are cheap and will fix your problem. There are plenty online, but locally you should find great deals as well. CompUSA and Best Buy always run special deals which will bring your cost to under $50, perhaps even under $20 (after rebate.) It is not like you are down, your phone and internet should work fine, and you should hardly notice an appreciatable difference, so you can take your time and perhaps get the best online deal for a router.
I also explained how to get into the gizmo and set up Dynamic Rate Limiting.
Then there is the modem swap with your internet provider for a newer model, perhaps.
The problem: Your particular modem and the gizmo don't like each other, bandwidth is throttled. Router fixes it. Typically Netgear and Dlink are the cheapest. The Linksys model WRT54G is said to have the best QoS (Quality of Service.)
You come here for advice. When given you don't seem interested in following it. Follow it and all should be well. Most any router will do (stay away from Belkin and Speedstream, though.) SR has been growing at the fastest rate ever recently. While there are few problems reported (I am actually amazed that they are running as good as they are with the rate of adoption they have right now) Level 3 tech support is probably stretched thin with their time, and I really don't see what they can tell you in this case. I am pretty sure they are not allowed to recommend a certain brand of router. They will probably recommend that you get one. IMO you really should, even if you did not have any type of issue, to separate your internet traffic from your phone traffic.
Since you are up and running you may want to look at this link for the best online deals on routers, starting at $18.49 (includes shipping and taxes) for a Dlink wireless router DI-524.
Lowest priced routers