When you want to transfer ringtones from old iPhone to new iPhone, you will come up with the sync problem when using iTunes. iTunes provides a "Transfer Purchases" function for the you to transfer purchased items back to iTunes library, but the customized ringtones cannot be transferred back. If you are not sure about how to use the "Transfer Purchases" function of iTunes, check out the following tutorial.
1. Start iTunes, and go to "Edit > Preference > Devices". Check "Prevent iPods, iPhones and iPads from syncing automatically".
connect iphone to itunes.
2. Connect your old iPhone to PC via USB cable, then double click on the iTunes icon to launch the iTunes app on your computer if it did not start automatically when your iPhone is connected to computer.
3. Click "File > Device > Transfer Purchases" at the upper corner. Then click "Apply" at the right bottom. iTunes will automatically sync the transferred items to iTunes library, and the purchased ringtones will be transferred back as well.
4. Then disconnect the old iPhone from computer. Connect your new iPhone to iTunes following the step 2. Click on your iPhone, click the "Tones" to open the ringtones tab. Check the box for "Sync Tones" and then check the boxes for the ringtones you want to sync. There are 2 options avaliable, check "All tones" to choose all the ringtone files to sync, or check "Selected tones" to select the specified ringtones that you want to transfer. Finally click "Sync" button in the bottom right corner of iTunes, it will start transferring ringtones from iPhone to iPhone via iTunes. The synced ringtone files will appear on your iPhone as a ringtone automatically.
If you want transfer customized ringtones between 2 iPhones, you may need a 3rd party tool on your computer. Feel free to read the following tutorial for details.
How to Transfer Ringtones from iPhone to iPhone