ZTE AVID Plus has built in Wifi Calling

[Phases]

• 5-inch LCD display with 854x480p screen resolution (196ppi pixel density)
• 5-megapixel rear camera; 2-megapixel front-facing camera
• 1.1GHz Snapdragon 210 quad-core processor
• 8GB internal storage; 1GB RAM
• microSD card storage for up to 32GB
• 2,300mAh battery
• Google Android 5.1 Lollipop software
• 4G LTE support for bands 2, 4 and 12

http://www.cnet.com/products/zte-avid-plus/

http://phandroid.com/2016/01/06/zte-grand-x3-avid-plus/