2 months later
Not knowing what type of ADSL router it is, I take at random the type I use (Netopia 3346) and I find the following link:
http://www.netopia.com/support/hardware/manuals/3346ENT_qsg.pdf
where we read on page 2-1:
One DSL WAN (or Line) cable
I think you can find the equivalent in a well-stocked supermarket. And if you need more detailed information, there are computer stores in French-speaking towns. Cordialement. Crigou.