Dispute and request validation in writing, sent certified return receipt requested, if you receive any contact from collection agencies.
Dispute Verizon's failure to terminate your service in writing directly with Verizon. It is important that this dispute be sent in writing to Verizon in a timely manner, so send it immediately, certified return receipt requested. Go to the FCC site for information on how to dispute telecom billing errors.
Request that Charter send you a letter indicating when they notified Verizon to terminate your service. File written complaints against Verizon with your state public utilities commission, state AG, and the FCC. In addition, contact your local DA's consumer protection department, and if you have a good consumer reporter with either a local TV station or newspaper, contact them.
Note that they have already screwed you, and are damaging your credit. Paying them now for their "error" will still leave their damaging entry on your credit reports. The most likely path to get it removed is to file all the above complaints, and let the sh*t hit the fan.
Verizon is in competition with Charter, and does not like losing customers to them. Whether you believe this was just an "oversight", or intentional, it benefits Verizon to both charge you for several additional months of "service" you don't need, and to screw you for your decision to leave, interfering with your ability to open accounts with competitors.
Intentionally failing to terminate your service when requested, so that they can both collect additional payments for unused services and damage the credit of ex-customers who leave them to switch to competitors, is an unfair trade practice.
