If your smartphone is showing "Mobile Network Not Available" error, don't panic and follow this simple guide to resolve this issue in two minutes.
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How to Block and Unblock Someone on Whatsapp on iPhone [iOS 9]
It's a step by step picture guide to block and unblock someone on Whatsapp running on iPhone 6s, 6, 6 plus, 5, 5s, 4s and 4. It works on iOS