Have you tried using the volume buttons on your phone? Sometimes the bluetooth on my iphone is turned down really low? Wonder if that could be the problem and you are over amplifying it using the headunit?

Start a call then press the volume button, it should show the bluetooth volume.

Jon Freelander 2, 2014 Dynamic SD4 Santorini Black

I think ive got it covered, but that does make sense so I will validate and let you know  2012 DISCOVERY 4 GS AUTO ORKNEY GREY
2012 FREELANDER 2 SD4 GS AUTO BALTIC BLUE - GONE
2011 FREELANDER 2 TD4 GS MANUAL BALI BLUE - GONE

I hope that sortes you out, I can confirm I have just tested my dads 3gs and my iphone 5 in his 2012 XS and the quality is great.

Jon Freelander 2, 2014 Dynamic SD4 Santorini Black

I have the same issue. Sadly cant turn up the volume on my blackberry when paired with bluetooth. Bberry says function not supported. (Coming) 2016 RR Evoque td4 180 HSE Dynamc
(Going) 2013 TD4 GS Auto - Santorini Black with black leather. Cold Climate, UK Lighting & Armrest Packs. Privacy Glass. LR Rubber Mats. Full size spare.
60 000 miles averaging 35mpg

(Gone) 2010 TD4 Manual GS - Stornoway Grey. Sold at 3yrs old & 64k miles. Final average consumption 39.2mpg.

Jon, have you tried it at 70mph? 2012 DISCOVERY 4 GS AUTO ORKNEY GREY
2012 FREELANDER 2 SD4 GS AUTO BALTIC BLUE - GONE
2011 FREELANDER 2 TD4 GS MANUAL BALI BLUE - GONE

I haven't but my dad has and has, he said it was much better than the quality his old car (mine now) which has a nokia jobby fitted to it.

Is there anyone locally that you could compare with?

Jon Freelander 2, 2014 Dynamic SD4 Santorini Black

nope, I am a band of one in these parts

I reckon as flylr says, I wont be able to change the volume from the handset... 2012 DISCOVERY 4 GS AUTO ORKNEY GREY
2012 FREELANDER 2 SD4 GS AUTO BALTIC BLUE - GONE
2011 FREELANDER 2 TD4 GS MANUAL BALI BLUE - GONE

Make sure volume is up full on iPhone when in a call, otherwise it just increases the ring volume. I am sure you know this anyway, I wonder if AVC affects Bluetooth volume?
Many threads ont' web regarding quiet iPhone Bluetooth connections.