This used to be a key differentiating feature between AT&T and Verizon. AT&T smartphones maintained a data connection when voice calls came in, whereas Verizon phones would drop offline for the duration of the voice call.
AT&T phones however dropped to 3G speeds during the voice call - slowing surfing way down, but at least you could research your dinner plans online while talking about them on the phone.
Some more recent Verizon smartphones have gotten around Verizon's network limitations by adding an entire second radio, so that the phone can maintain a data connection and a voice connection at the same time. Apple however has never bothered to add this extra cost and complexity to the Verizon iPhone.
Eventually VoLTE (Voice Over LTE) will come along - unifying voice and data communication channels. When that happens, these limitations will be a thing of the past. But for now - if voice calls are important to you, don't assume you can use your smartphone as your primary data device.