Verizon breaches contract

We had a large increase in our bill after our last contract expired. I called Verizon and they agreed to give me a lower price. After about a year under the new 2 year contract, my bill went up to about what it was before I called Verizon. I contacted Verizon again and they say a charge had been overlooked. In my mind, this is a breach of contract. We agreed to a certain price and they have changed the charges in the middle of our contract. Who can I speak to about this? The billing department was not helpful.