Linksys SPA 3102 Review
As the review says "Apparently the wiring is different..."
The RJ11 has 6 contact positions, and I notice some of the newer cables only wire up the middle 2 contacts. The phone used in the review must have been wired differently.
I suspect that if you are using a North Amercan purchased phone, you will not need any adaptor, and the standard cable that comes with the phone should work fine.