Will it affect my credit which is excellent, if I change providers and do not pay cancellation fee?

File a complaint with the FCC. You can file this online using a complaint form. You can find this by googling, since we are not allowed to post links. Ask the officials at the FCC to help you get out of this. In the future, you may want to go with phones that do not require a contract. Contracts seem generally not in the best interest of the customers. Without a contract, you have the benefit of being able to switch to any new plan as it comes along and the company has the incentive to create attractive new plans. I cannot imagine being locked into the same plan for 2 years.

Any failure to pay a bill will probably end up on your credit score. However, you can challenge that entry and correct the report. You might also contact the public service commission or whichever government agency that regulates cell phone service in your state and ask them to look into the problem.