Do I need a separate telephone line to use DSL ?

In the majority of circumstances, the answer is no; a single phone line is all that is required.

ADSL operates at a different set of frequencies than standard voice phones. That difference in frequencies allows for both voice and IP traffic to co-exist on the same physical phone line. In most cases, the service provider offers DSL line filters that plug into wall jacks to separate voice and DSL traffic interference.

Note that there are some different types of DSL, such as SDSL that may require a separate phone line.