Q: HD Voice is not working although supported by operator

I upgraded from 3GS to 5 recently.

My operator supports HD Voice calling.

When I tried it from my iPhone 5 to any other phone which supports HD Voice, it was just regular call.

In order to test we we in the same place several persons with HD Voice devices with the same operator.

Between other devices it was OK.

Putting others SIM cards into my phone didn't help.

What else can I check? Any indicators, settings etc.?

Is it iOS 6.1.3 which is installed or device itself?

Wha could be done?

Thanks for asnwering.