It may be better to take the car to the shop - but when you schedule the appointment to do this, use your system in the same way you described above. Do this in a way so it's safe, but let those at the shop hear what you're hearing - its' more effective when they experience it versus you trying to describe it and helps with the diagnosis.
Whatever you do to set this up, call the shop and talk to them thru this system - and they may know more of what to do.
An Echo - from overseas - we can't really offer much help due to the issues of you being in the UK versus what we have over here.
And to be honest, I've not allowed my vehicle to "process" any phone call, I do it all thru the phone and keep the hands free headset or use the phones "speakerphone" option to process the calls as if I was in an office using the offices speakerphone.
It works better that way for everyone that rides in my car - easier for them to take calls personally.
Now with road noise being what it is these days, the (newer smart)phones speaker tends to process the noise and noise gate the effects out better than the on-board systems can - this includes my wife's Nissan - she has similar issues of volume to noise level and has kept to her own phone due to ambient noises over-riding the voices and incoming chatter on the system due to it's own "noise gating" it tries to quiet out the exterior noises.
