For years I have had people ask why it is so hard to get a billing issue resolved when calling into an 800 number? This is the best loaded question next to "Do I look fat in this?" question. Several factors contribute to why it goes from something simple to an epic quest.
The first thing you want to do is make sure you have the account information up to date. This means phone numbers, authorized users, the whole deal. Billing calls are subject to so many necessary verification that even one off can cost the call to go south. I would advise to call in whenever you get a moment and make sure that is all correct. I myself will call my utilities and other bills just to make sure all is well every other month or so.
Once you get past the verification part, ask the rep you are talking to if they can see what you see for the bill. Most will answer yes but they will be looking at a ledger screen most of the time. If you ask them to look at the copy you receive as a customer it will make it easier for you. A line item you see will not necessarily be the same the rep has when looking at a ledger screen.
If you are calling with a lost payment, have all the information there. By information I mean everything you can think of and then some. You have to remember that you are looking for a LOST payment. The changes of the place you are calling having the information is slim to none. If you paid by cash, keep the receipt until you can verify from the company it has been applied. I recommend keeping it until you get the next bill showing it has been applied. Cash is not easy to travel and I ha e yet to see a company just credit an account because someone said they paid in cash but doesn't have the receipt. Electronic payments are a little easier. Most places give you a confirmation number, an email, or both. Keep those as well until you see the item clearS out of your bank account. Most businesses will want the confirmation numbers from the bank as well as a copy of the cleared item. Without that information, it become a needle in a hay stack. Once again, documentation is key to your success on these types of calls.
I hope this information has been helpful or if not anything serves as a refresher. In this age of everything done online, you still need to keep all your personal information up to date and copies of transactions handy. You never know when you may have to pick up the phone and speak to a human.