Hippo said:

Go into:
Setting \ Connections \ Mobile netwurks

Hhen click on Netwurk connection

Make sure its set to 4g 3g 2g auto connect

Eye is with 3 anorl. Via Smarty. Unlimited calls and text, wiv 6gb of internet. All fer 6 sovs per month. Other packages available.

Click to expand...

It IS set to 4g 3g 2g auto connect, BUT it isn't 4gVoLTE compatible, which is why when I try to make a phone call with it, it just sits with "Calling" showing but the other phone which is right next to me doesn't ring.
But I can send it a text.
Here is a search thing for VoLTE compatibility from SMARTY, as you use that.
The very first Phone on the list of phones which is NOT compatible is mine, Alcaltel 3.