6610 speech codecs... someone knows something?

[Thanh]

Hello
I have read on the web that setting a phone to "half rate codec" would save 30% battery, i.e. longer talktime. Everything i found was about older phones, but trying *4720# worked also on my 6610. I made a "test-call" to my friend and didn't find any difference in sound quality between this codec and the previous used "Enhanced full rate". Now, i am of course not sure if my network supports EFR at all, as there was no difference, so my questions are:
Does using HRC *really* save battery? Even when the network eventually does not support EFR?
and...
When i turn off the phone and on again, will it still be in HRC mode, or does it revert to EFR, which seems to be the "sandard"?
Any advice on this would be highly appreciated
Thanks in advance.....
your Thanh

[liteFun]

FR & HR works.
i think it's like sampling frequency, HR like 11025Hz, FR 22050Hz, EFR little bit better than FR, like 26000Hz.
that's the way i'm thinking that. because i'm using EFR, my friends use EFR, sp said EFR works on their network, and there's too less difference between FR & EFR to hear. but between HR & FR it's more.