This solution only works if you once had Yahoo Messenger on your iPhone. Access it from your cloud by going to the App Store and choosing updates. Click on purchased at the top of the list and you should see a list of apps that you have purchased. Scroll down until you find Yahoo Messenger and click it to reload to your phone. I found this solution by accident. Have searched the web for an answer and didn't find anything. Hope this helps. Not sure how much longer it will work since it is being phased out. I have an iPhone 4s so hoping this works for you too.