Ringtones and audiobooks can only be downloaded once. Transfer it from your iPad to iTunes, then sync it to your phone.
Downloading (using iOS or computer) past purchases from the App Store, iBookstore, and iTunes Store - http://support.apple.com/kb/ht2519 - enabled with iTunes 10.3 and newer; not all media formats are available in all countries (see: iTunes in the Cloud and iTunes Match Availability by Country - http://support.apple.com/kb/HT5085); apps, books (not audiobooks), music, t.v. shows, and movies (some - not all studios have permitted this). Downloading previously purchased movies and TV shows requires iTunes 10.6 or later. Discontinued items not available. For items not included in the iCloud list (e.g., ringtones, audiobooks), or locations or computer systems where iCloud is not (yet?) available, you only get one download per fee paid. Apple notes it is your responsibility to back up your purchases.
Select the iTunes Store on the left side of iTunes.
Click on Purchased on the right side under Quick Links.
You can re-download your available previous purchases.
If the above doesn't work, it is reported iTunes Support permitting a second download when asked nicely, but they are in no way obligated to do this. Go to https://expresslane.apple.com ; select 'iTunes' in the first column; 'iTunes Store' in the second column; third column click 'Purchases, Billing & Redemption' and proceed from there.