This video tutorial from xTheEnforcerx1 shows how to import custom SMS tones to your iPhone 3G using SSH.
Make sure iPhone is connected to your computer. Run SSH client to view all files and folders on your iPhone device.
First, you need to convert your audio file into SMS tone.
Open iTunes, Preferences-Import Settings. Choose import using AIFF format option.
From iTunes library choose audio file you want to convert. Right click it and choose Create AIFF version. Then drag AIFF file to your Desktop.
Right click it again and select Get Info option.
Rename file's extension to .caf.
Click Use .caf.
Now in SSH browse to System-Library-Audio-UiSounds. Rename your converted file to sms-received4.caf and drag into UiSounds folder. Close SSH client and disconnect iPhone.
Now you can choose your new SMS tone using iPhone settings.