A codec is a way of compressing speech. More compression requires less
bandwidth but can sound worse.

Typically, a phone will have a preferred codec, but will work with others.
For example, someone might callyou with a low bandwidth codec because that's
all the bandwidth they can get and currently, you would reject their call.

So I'd recommend removing the "only use the preferred" option. The codec
you have is the highest bandwidth codec you have anyway (probably) so having
this "just this codec" option isn't helping you.

Paul DS