Calls start "HD" but soon go back to standard quality

Hi,

I'm in Tokai with a phone on Vodacom that supports HD voice (AMR wideband, right). My brother lives quite nearby.

I notice that quite often when I phone him the call starts with "HD" on my display and sounds great. But quite quickly the HD disappears and you can hear that the call goes back to normal quality.

Have others experienced the same? Does anyone in Cape Town get "HD" calls consistently?

Thanks,
Elbow