Try this

Turn International CDMA On/Off - Apple® iPhone 5

[Device-Specific Instructions]

Note Turning off International CDMA while in another country may improve poor messaging or data performance on a GSM network.
1.From the Home screen, navigate: Settings > Cellular > Roaming.
2.Ensure the Voice Roaming switch is turned on Switch On (in the green position).
3.Tap the International CDMA switch to turn on Switch On or off Switch Off.

or this link

https://support.apple.com/en-us/HT201643