Your best bet is to delete the contact out of the phone, and then create a new contact for your sister.
I have a Samsung S3500 slider mobile phone and works great. By mistake,however,I added the wrong phone number to the "add to reject list" feature. Now when I call my sister she does not hear a ring tone,instead my call gets to her voice mail.
I cannot find how to undo this and make it work in a regular way.
Can anyone help me? This is most annoying.