SEO problems? No longer be liked by GG.

[callback]

I have to ask myself if I have done anything wrong to make my site drop deeply from #1 to the bottom of GG (invisible).
I target on two services: "callback" and "toll free".
For over two years, the main page http://bestcallback.net was at #1 on keywords
"callback service," and the pl. services, and "international callback." Now two months ago, it suddenly dropped to #65, and now backed to #24 and #32.

The other URL, which is within the site:
http://bestcallback.net/toll_free_service_2002.htm used to rank anywhere on the first page on many keywords such as "cheap toll free," "toll free 800" and so on. And now none of these keywords will bring me to even top 100.

I have done SEO on each page for long time and every page is fully optimized as much as I can tell. I can't figure out why GG does like them any more.

Some nice guys told me I lack of inbound links -- that's true, but I hadn't got them for long and it never bothered me getting high rankings. Why now?

I do have any site presenting almost the same contents. I don't know if that duplication caused the trouble. It is http://bestcallservice.com . This site is doing so so with Yahoo!. Could this is the major problem?

Thanks a million for looking over my issue.