Muted individual contact; do not receive notifications on messages
I honestly don't know what I did, but one of my contacts has basically become muted. I do not get any type of notification when he sends me a text message and even in the text message list, there is the mute symbol next to his name (bell with diagonal slash). How can I remove this? He is someone with whom I text regularly and I can't continue to be unaware when I receive messages from him.
Thank you all for your help
iPhone 4S, iOS 5.0.1