All of the major carriers are migrating to LTE networks. The iPhone 5 is an LTE capable phone, but not for voice calls. Voice calls in LTE utilize VoLTE ( Voice over LTE), a type of VOIP. The iPhone 5 is not VoLTE capable. That didn’t come out until the 6. So, when you currently place or receive a call, your carrier will shift you over to their 3G network for regular old circuit switched phone call. The good news is that those 3G networks will be around for at least a few more years. Although the bandwidth allotted for 3G use is shrinking to the minimum necessary.