I didn't add this string originally, but Borja suggest that we can have a temp string here and refine the string in .
Basically the string should be the same as message text in https://www.dropbox.com/s/mrtjwvzk6xgoc02/HTML5_SMS-MMSUserStorySpecifications_20130503_V8.0.pdf p.38 , but the wording might also need refine because the sender of the not downloaded notification message could be operator number. I already notify Ayman about this possible behavior, but the wireframe didn't update yet. I'll ping him again in https://bugzilla.mozilla.org/show_bug.cgi?id=840087 and we can refine the wording in this one.