If you want to assign dedicated ringtone for a contact on your Windows Phone 7 mobile phone then follow the process written below.
Assign / Change Ringtone for a Contact
Step-1
On “Start” screen, tap “People” and flick to “All”
Step-2
Tap on the required contact to which you want to assign ringtone and then tap “Edit”
Step-3
Tap “Ringtone” and select the ringtone which you want to assign to contact then tap “Save”
Done! Your selected ringtone will be assigned to that specific contact.
Note: To assign separate ringtone for each of your contact on Windows Phone 7 device, you have to repeat the process for each one individually.
See more Windows Phone 7 tips