There IS a toll-free Mileage Plus customer service phone number for all Premier members (including basic Premier members who don't fly enough for 1K and Exec). The phone number is on the back of the Premier card.

Overall, I have been really impressed with United's customer service. Occasionally there might be some agents with nasty attitude. The solution to that is take down the person's name and work location (ask them for it), and write a letter (e-mail on the United website www.ual.com or snailmail) to let them know. If you have an elite status, mention that in the letter and include your mileage plus number to increase your leverage and chance for a response. Often UA will respond with an apology letter and (occasionally) a "we're sorry" discount gift certificate good for future UA flights.