I started encountered an issue around two months ago where I'd estimate probably 10% of calls had this issue. It seemed a lot worse around a month ago I've been away for 10 days in the US and have made very few calls since I've been back but the issue still exists.
During call setup on an AMR-WB (HD audio) supported handset the call is being setup to use AMR-WB. When the call starts to ring its presumably being transcoded (which is exactly what it sounds like to me) which results in a garbled sounding audio due to the transcoding - my pick would be the G.711a off-net call possibly being transcoded twice to AMR and then AMR-WB. After around 10 seconds the codec is changed mid call back to AMR and audio clarity returns. If the call is answered quickly you'll get garbled sounding audio from the other party, if it's still ringing you'll hear the ring tone sound clearer.
This only happens with off-net calls, it does not happen with on-net AMR or AMR-WB calls.