Companies usually have ISDN connections so their telephone number can have many extensions. Each extension is assigned to one person in the organization which makes it possible to call that person directly.
As you know we are living in a fast moving world. It's the world of telecommunications. Almost everyone today has their own mobile phone. In Germany you call them "handy" but no-one in England or the United States would understand what you mean by that. In England, we say "mobile phone". "Call me on my mobile!" "Have my mobile number." In America they use the term "cell phone". But the word "handy" is an adjective and it means "easy to handle", "convenient".