Had my first issue today with my S4 it is a week old today. Anyhow my wife texted me and asked me to call her. So I hit the dial button and just an error unable to access network while I am home it uses my fios high speed router to do wifi calling and so far it has been working fine. So now I changed the prefs. to use wifi if network is unavailable. I had two laptops on the router when this happened one was on wifi. Maybe just a fluk?
Other then that the phone is great,