I'm pretty sure it's a provider settings for the iPhone. There is no setting on the iPhone that I know of. Check with your provider to see if there is some voice menu you can use to change this setting.
For the iPad it is definitely on the device, since there is no phone service, but if there is anyway to change it I've never come across it.
What settings exist for ringtone are under Settings > Sounds on the iPhone and Settings > General > Sounds on the iPad.