What Carrier device is it and are you currently on the Same Carrier if not which one are you currently using. Wifi calling is a Carrier features so on some Carrier won't let it work if the device isn't connected to its carrier. Also try this, Press Force Stop and tap on storage and tap on clear Cache and pull down status bar and turn on wifi calling and see if it works.
In order to access wifi calling on T-Mobile (Parent of Metro) you need one of the following:: T Mobile branded phone or unlocked Galaxy S/Note. Phones running firmware from ATT, VZ, and Sprint won't work with TMO/PCS' VoWifi feature.
Ya, I don't think At&t allows to use At&t wifi Calling if you are not in their Service. Try to talk with metro Pc and see if they can add or give that service, if not make sure with At&t if they can do anything, if not you won't be able to use it.