The telephone you already have is a standard landline phone plugged into a standard telephone socket?
My Virgin cable comes through the wall into a box. From the box another cable goes to a cable splitter and plugs into cable wireless router and Tivo box.
From the box is another smaller cable which goes into a telephone socket from whence you can plug in a standard landline telephone. As the extension lead will be wired the same way round but with male/female connectors, I do not foresee a problem.