I have a Nexus 4. When I add a new contact, say from a friend that has texted me so that I have their number only, I go through the process in 'contacts' and put in their name. This then shows in messages as my friends name. All good. Then a little while later this is lost and the name reverts back to just showing their number. The only way I seem to be able to store numbers permanently is if they have an email address as well. Why is this and what can I do about it?