I want to transfer some custom ringtones to my iPhone 4s. They are in mp3 format. Is it possible to put custom ringtones on the phone?

The ringtone format for the iPhone is m4r. I don't think mp3 files will work. you will probably need to convert them. If you have a Mac with Garageband installed, you can easily convert the MP3 to an iPhone ringtone and have it automatically import into iTunes for syncing.

iRinger is a great free program for turning MP3 files into 30s ringtones. The only problem is that the free version does spam with some ads and does limit you to a 30s clip. It's worked for me for as long as I've had an iPhone though, so I do recommend it. It also automatically sticks them into your iTunes Ringtone library so they automatically sync with the phone.

I'm glad I found this thread, I'm really liking this iRinger app. It's pretty easy to use, and supports multiple file formats. Thirty seconds is plenty to me for ringtones, and I always expect a few ads with good apps. I guess I just feel that having to deal with ads is fair since I'm getting something useful for free.