That is most likely because your in area with bad signal, the only way to check if it's your phone is to put your sim in another phone and check if the same happens or go to an area which you know has full signal and check - if the same happens here on both phones, you may need a sim replacement.
Also you will need to monitor this behaviour over a period of time.
Lastly 4g is currently only for data not voice, simply put voice only goes over 3g and 2g until Voice over LTE (VoLTE) is rolled + activated in your area.
06-10-2015 02:23 PM