Samsung S3500 add to reject list feature

I have a Samsung S3500 slider cell phone which I like very much. By mistake, I added the wrong phone number to the "add to reject list" feature; now I cannot find the way of "unrejecting" this number to make it work in a regular way again.
When I call my sister she does not hear any ring tone in her phone, although my call gets to her voice mail.
Can someone be so kind and help. I'ts very annoying.
Thank you!