The telephone wire is made up of a flexible plastic (usually PVC along with some colouring additives) material moulded along with the conductors inside This is the cable. This cable is again given the helical shape by raising its temperature to approximately 120 degrees Celsius, the point where the cable does not melt but the polymer configuration easily gets altered This changes the physical form. It is then cooled to ambient temperature where the shape is retained, however, with the inherent elastic flexibility of the material. Thus the cable can be stretched easily to a helix of larger pitch and when released it resumes the original helical shape instead of getting untwisted. Of course, if the cable is held stretched for a while in an ambience of raised temperature, it would lose its shape.
Source: thehindu.com
