[Q] How to enable HD Voice aka WB-AMR? topic

Hi,
I found out how to get into the service mode and now I see the WB AMR Settings. There I have a selection of 1-4 ...

[1] UMTS WB ENABLE (*)
[2] UMTS WB DISABLE ( )
[3] GSM WB ENABLE ( )
[4] GSM WB DISABLE (*)

I would like to enable GSM WB. The problem is, that when I press 3 all I see is another windows with a value "3" but nothing else happens.
What am I missing? How to I enable that option?

Anyone?