OK, so you were talking about activating a new line of service. Initially, you were talking about buying a phone at full-pop and activating it on an already active line while still increaseing the contract for two years. That does not happen. Now adding a line and activating an old phone on a new line, depending on the plan, will give you a new 2 year agreement, that i can accept.
Your initial comment made it sound like an activation of any phone, new or old, would automatically extend the contract another two years on an ACTIVE line. The new line of service is always two years UNLESS you can acquire the "No contract" plan which i believe no longer exists.