What's the best phone cable for DSL?
Use a twisted pair wire instead of the parallel satin-coat wire normally used to connect telephones to wall-jacks.  You may have to make your own - this is easy if you have the tools and connectors.

I purchase a lot of 3-pair telco cable - the wire guys often refer to it as "Cat-3" wire.  This is 3 twisted pairs inside a pvc jacket.  However, one pair of Cat-5 should work well as well.

The reason I would use Cat-3 cable over Cat-5 is that the number of twists per foot for Cat-3 is similar to the number of twists per foot of the cable between your premises and the telco central office.  Cat-5 has many more twists per foot than Cat-3.

Regardless, this is quick and easy for you to test.