A little tip about cell phone billing, that I frequently passed onto nice customers, because who doesn't like to save a little bit of money? Now this only applies to Cingular, since that is my only experience!
When call in regarding a bill issue, do everything you can to be nice! Sure you may be peeved that your bill is incorrect or that you have extraneous charges, but acknowledge the fact that the person helping you isn't responsible and you appreciate their help. Not only are you just being a good person, but it will really help you out....
Ok so let's say you have a text messaging plan that allows 200 text messages- but you go over and now are being charged an extra $15. Say that you thought you enrolled in a higher plan and really didn't know! Chances are that they will bump up your text messaging plan to cover the extraneous texts, which will save you major money, and they will make it retroactive so you don't have to pay the fees. Pay your newly adjusted bill, then just simply reduce your text messaging plan (you can do this on the cingular website). You just saved yourself from paying extra!!
Another: Let's say you got on the internet a lot on your phone, and you unfortunately do not have a multimedia plan. Now you've just caused yourself to have really expensive usage charges. All you have to do is tell the CSR that you did not realize you didn't have a media package, and they will do the same thing for you that I described above ^^. If you're really daring, you could also say that your toddler, or random objects in your purse are responsible for accessing the internet. Also, some phones have an internet button that allows instant access, you could blame it on that darn button for its poor interface

. If you've been nice during the entire conversation, they will probably just simply remove the charges.
A super awesome tip about replacements: I did this just recently and had no problems (neither did my customers who did this). I dropped my phone and it began running hot, not keeping a charge etc. So, if you dropped your phone and it has suddenly stopped working properly, but there appears to be no physical damage, you can still get it replaced.. for free!!! Call the hotline and say that your phone has stopped working and you've tried to troubleshoot it.
Do not tell the csr that you have dropped the phone!!!! The CSR will then connect you to the warranty hotline where they will ask you some basic questions to make sure there isn't water damage, then they will authorize a new phone to be sent to you. If you also mention that it is a business phone and you desperately need it, you can get upgraded to next day air for free. Now this will only work if you have had your phone for a year or less, but it's a great way to avoid the insurance replacement fee.
Any other questions? I'll be happy to answer them!