If you want a better app to do that, I use an app called "WhoIsIt" from the store, it will let you set different ringtones for different contacts, even for texts and emails, it will also let you set a different ringtone for "unknown callers" (not in your contacts list) ... and even VIP callers, where even if your phone is in silent mode it will ring normally for very important contacts. It's a great app