I've been with Sprint for years and years and never had any problems with them. But I discovered last week that it was because I never had to call Customer Service.
Last month Sprint changed my account number so I made sure to go to my online banking and change the account number there before I made the payment. I made the payment and forgot about it. Then last week I got two statements from them. One for the old account number with a credit balance (my last payment) and another for the new account number which was past due. I checked back with the bank and the payment definitely went with the new account number.
I tried going on line but couldn't remember my username or password and even going through the "don't remember" links I couldn't get on. I could reset my password but then the site wouldn't accept the username (which they had e-mailed to me moments before) so I tried the "contact us" button to complain but I had to log on to send them a message.

So I called in a payment (in my mind past due is apparently one step away from serial killer) and went to bed.
The next day at work I tried calling and discovered their labyrinth of customer service. What happened to those despicable phone trees where you have to listen to dozens of options before finding the one that might be what you want? At least then you had a chance. Now they have this evil robot woman who pretends to be your friend ("just say what you need") but misunderstands anything you say. All of the things she suggested didn't fit and when I did say something she said "okay, technical support" which didn't sound right at all so I hung up but actually pressed the button too long and turned the phone off.
It was a quiet afternoon at work so I tried again a little later and somehow managed to get to the "contact a human" option where they told me it would be a 15 or 20 minute wait so I sat there listening to the most horrendous staticky hip hop music. I waited it out and after talking to two different people they said they would transfer the credit to the new account number.
By the end of the week I'd forgotten all about it until I saw this thread

...but I was finally able to log in to their website so I shouldn't have to deal with evil robot woman ever again.