If you change your rate plan with a cell phone carrier, can that carrier extend your contract 2 years without disclosing the extension and getting your consent?

No, your contract may not be extended without your consent. You consent can be implicit, however, not explicit, so long as the extension had been disclosed (e.g. as part of the new service agreement; in the advertising or marketing materials; etc.). If the extenstion were disclosed to you--which does not have to require it being explicitly pointed out to you; it is enough that it is in the materials you were presented with or signed--then from the fact that agreed to the new plan, you can be taken to have consented to the term extension that was part of it.