They didnt even call my references so cant say for sure but heard its pretty basic. Just make sure you are on the same page with since how long you know each other , what relationship you have (cant be family member), where you live and work.
They usually call when you request urgent processing, I once had attended a call for coworker reference.
They just ask 2-3 questions about person and your relation.
I had mine made urgent but took my guarantor with me, it was simple process.
Hi,
They called my Guarantor and references. Asked them how they came to know me. How does he/she look? (wear glasses, lean/fat, short/tall etc.,) How long have they known me?
I requested urgent processing and my guarantor or reference persons were not contacted. They might be doing it on random basis or when they would like to know more about the person out of any concern they might have about the identity of the applicant.