Network settings like VoLTE

Not all carriers support Voice over LTE.

Check here to see if yours does: https://support.apple.com/en-us/HT204039

If they do, call them. Your account needs to be properly provisioned for VoLTE to work.

About the LTE options on your iPhone

The cellular-data options on your iPhone might vary based on your carrier's network.

If your carrier supports VoLTE

On your iPhone, go to Settings > Cellular > Cellular Data Options and tap Enable LTE or Settings > Mobile Data and tap Enable LTE. If your carrier supports Voice over LTE (VoLTE), you'll see these options:

• Off: Turns off LTE
• Voice & Data: Allows voice calls and cellular-data use over LTE 
• Data Only: Allows cellular-data use, but not voice calls over LTE

If your carrier doesn't support VoLTE

If your carrier doesn't support VoLTE and is a GSM carrier, you'll see one of these options:

• LTE or 4G: Allows cellular-data use over the LTE or 4G network when available.
• 3G: Allows cellular-data use over the 3G network when available.
• 2G: Allows cellular-data use over the 2G network when available.

Depending on the wireless carrier, 4G can indicate either a UMTS or LTE connection. If your carrier doesn't support VoLTE and is a CDMA carrier, you'll only see an option to Enable LTE. This allows cellular-data use over the LTE network when available. 

About the LTE options on your iPhone - Apple Support

https://support.apple.com/en-us/HT203124