I assume you want to set the song to play whenever that contact is calling you. To do that you will need to convert the song to a ringtone.There are many ways convert a ringtone. (Due: Note the song cannot have DMI on it. If it's purchased recently from iTunes it should not have it) I'm using a iPhone with iOS 6.1 and iTunes 11.01 so if this does not work make sure you have the same or latest software updates.
First you will need to plug your iPhone into a computer
that you regularly sync with, on the account, and with iTunes open and able to see your iPhone.
and install this free app on your iPhone called Ringtone Converter - Make Unlimited Free Ringtones
Once the app is open
. A windows with instructions will appear. You may read the instruction or may not. Either way dismissed once your done. Then click the Pick Song
button to choose the iTunes song you want.
A window will drop up and you now scroll through and find your song.
Now move the slider, or to finely move the time used the gray dial below the slider, to choose where in the song you want the ringtone to begin.
Their is a play button that you can click to hear where that spot is. (Note: the app automatically makes the ringtone end after 30 sec)
Once you find the spot click convert.
It will then convert the song. Once you see the Success box you know all is well and you can move to the next step. (Note: If the screen starts flashing, just exit the app and move on)
Now this is where you will use iTunes. Make sure iTunes is open (mac or PC) and connected. Then click on the iPhone.
Then click on the apps button
Then scroll down
to the bottom of the page.
You should see the Converter
app nowClick on it.
You should now see your ringtone.Drag it onto your desktop, or click the save to
button to save it to your desktop
Once saved on your desktop. Click the done button
Step 13:Add the ring-tone to your Library
by dragging into your music library or click Add to Library...
Step 14:Click on your iPhone
Then Click the Tones button
the Sync Tones
and check the All tones
Then click Sync
Then wait for it to sync with can take a while or be very short depending on the last time you synced.
Now your ringtone is in the iPhone and you can go to your contacts app (or Phone App) select the contactclick edit
click on your new ringtone (which should be on the Top of all the ringtones)Click saveClick done
You now have completed takeing a iTunes songConverting it into a ringtoneand setting to your contacts
Why did apple make this so hard? Because they rather you buy ringtones from them instead of using their songs as ringtones.