What signal strength does your phone show? Is your phone capable of voLTE? If not, you might consider disabling LTE on your device. You can do so by setting your network selection to GSM/WCDMA(UMTS) or 2G/3G. With the exception of voLTE and Wifi-calling, voice calls are transmitted via the '3G' networks. It's possible that there is a delay in your phone switching networks causing your phone to miss calls.