Hi,
Although the DSL standards are pretty much the same around the world, I would NOT advise your friend to bring his DSL router from US to Turkey because, The lines are actually still 2 wire analogue. This means to be able to connect a DSL router to a line, it still requires a DSL analogue to digital filter (we call it Splitter) added to the line to separate digital signals from analogue voice signals.
That's the standard for all ADSL out there. Most US providers drop a few wall splitters with the install kit. They also have the whole house splitter they install at the MPOE.
I forgot about another hitch in the getup. Lots of different providers have different VP/VC settings. Most in the US use 0/35, some use 8/35. There is no guarantee that you will get the info correctly.
IOW, don't bother bringing US equipment...
