It is important to understand that most law systems (including Islamic Law) target universal principles and not the modes and means. For example, murdering an innocent person is a crime in itself and is not dependent upon whether the killer used a sword or a gun. If we take your example, then gun was not there in Prophet (SAW)'s era; does this allow people to kill others with guns?
So given that you understand the point, using someone else expense to fulfill some of your needs without his/her consent is unlawful in the first place. If you haven't taken permission from your office management about personal use of the phone, then you are directly increasing office's expenditure (which someone will be paying, right?) through your personal use without their consent.
If you have already done that, you should bring this into the knowledge of office management. They'll probably not mind it. If they do, you should offer them compensation (deduction from your salary or something else) by inspecting past phone bills.