If you're experiencing any issues or concerns regarding your MileagePlus account, your first stop should be the page that contains the contact information for MileagePlus. You can learn how to join, ask for lost miles, or even change your account name from there. But if you have a specific question or problem, you can call the united airlines toll free number 1-800-780-0235 anywhere in the U.S. or Canada. You can find a local customer service number here if you're calling elsewhere. In addition to that, you have the choice of sending in a contact form.