Devices that support LTE but do not support VoLTE (making phone calls though LTE) in a specific operator's network just before a phone call do connect to 2G or 3G network in order to make a call. It is being called Circuit-Switch Fallback (CSFB). After that they (should) switch back to the LTE network connection.
Devices which support both LTE and VoLTE in a specific operator's network during a phone call use LTE network to make a call.