I just made that call.
This is what it entails:
An automated voice welcomes you and asks you to state your age. Then he (the UK voice is a chap) asks you to enter your age using the ‘phone’s number pad.
Then he asks you take a deep breath and say, “Aaaaaaaaah” for as long as you can. He asks you to do this two or three times.
Next, he asks you to say “yes” and “no”; he then asks a few details such as whether you have been diagnosed with Parkinson’s or any voice disorder.
After that, he asks you to repeat a series of words and sentences after him. There are a few bits about a fictional grandfather. As I recall, there were no words in which the vowel sounds would vary greatly from north to south.*
Finally, you get thanked for your call and given a reference number, which you can – if you wish – use to request feedback on the PVI site. You will need to give them an e-mail address if you do this.
I requested feedback, but it hasn’t arrived yet.
*For those outside of the UK, there are distinct regional variations in the way some vowels are sounded; one of the most typical is the “a” sound in “path”. I, a midlander, pronounce this with a flat “a”; many southerners use a broad “a”, which can be readily represented by inserting an “r”: parth. I can’t think of any reasonable way of representing the short “a” except for the correct spelling; read into that what you will.