Which type of health insurance provider generally does not allow patients to obtain services outside its preferred network of doctors and specialists

Preferred provider organization (PPO)  

A preferred provider organization (PPO) is a medical care arrangement, and a type of health insurance in which health care providers and facilities agree with an insurer to provide health care services to the insurer’s clients at reduced rates. PPO participants are allowed to obtain only the services of any health care provider within their network.  PPO plans are the most common, and the oldest healthcare plan because of their lower cost and flexibility