So I recall a while back using my wife's Spark supplied Samsung Galaxy S7 and making a call to another mobile and being amazed at how clear it was.

I was using an older Galaxy Note 2 (Parallel import) on skinny and the call quality was a bit ropey at times.

Just upgraded to a Galaxy A5 (2017) which is also a parallel import phone.

Have noticed that call quality is not always that clear to be honest.

Tested calling my wifes Spark supplied S7 to my Skinny A5 - so the network shouldn't be the issue.

I dont know how you check if HD Voice is working, but I found this note on a Skinny support page:

https://www.skinnydirect.co.nz/coverage/

HD Voice - Spark has rolled out HD Voice capability across the country, so you can make and receive great sounding calls no matter where you are. Both you and the person you are calling must have Spark or Skinny-supplied compatible phones, otherwise you may not experience HD voice quality.

Does this mean that a parallel import phone may not actually be using HD voice?