A true telephoto lens can actually be shorter than its focal length. The design of the lens moves the nodal point - the point from which one measures the focal length - forward to a point nearer the front of the lens. It is actually possible to have a nodal point that is effectively outside of the lens itself.
That sort of design makes for a smaller lens, but of course involves some other compromises.
Wide angle lenses often involve something similar (but in reverse). A retro-focus lens is designed to move the nodal point back, so as to allow it to be closer to the film surface then the lens elements themselves.
You cannot do this with a single element lens, but once you start using multiple elements of different shapes and types ....