No route found

My elderly neighbours' iPhone fails to make calls after dialling the number he receives 'no route found' then the call terminates

All the information I have.

I phone 6 (Probally)

iOS needs at least 1 update

Reset Network

Restarted device.

Located in Tawa area.

I'm assured there is credit on the phone.

Shows connected to 2 degrees plenty of bars 4G

tried switching off wifi calling

any ideas.