I have tried converting two CSV files, both of which pass the XML validation test. The only difference I can see from the GPX file I exported from the Volvo is that in the phone number the brackets around the country and area codes have disappeared in your version. In the Volvo implementation, the number appears as +(33)-(3)-86939386, but in the output when converting the number from CSV into a new GPX with your tool I get 33-3-86939386 without the brackets. I made sure to include the leading >.
I think the Volvo system needs these in order to dial the number correctly from a mobile phone connected via Bluetooth, so the extended functionality would not work. Is this fixable? I'll hold back checking this in the car until you can clarify this.