Connection error or incorrect mmi code

[JRS]

When using a *# code to improve my indoor signal I get connection error or incorrect mmi code ... 

[JRS]

I'm having to get it repaired or atleast attempt it

[Robin999]

Try resetting your network settings first

Settings>General management>Reset>Reset mobile network settings

[JRS]

I've done all possible changes

[Robin999]

You need to contact your network provider and ask them to deactivate wifi calling.