When shading, there are a lot of tricks involved. There is smoothing information in the mesh that makes things look good. And that is the way it’s supposed to be - there is nothing wrong with the mesh.
The base of your telephone is less than 5 mm wide - having 1 mm thick walls will create self-intersecting meshes. Are you sure your dimensions are correct?
At any rate, in this thread, @laurent_delrieu describes a way of dealing with offsetting meshes: