Its totally up-to VO , When according to documentation provided to him , if he/she is not convinced or if he/she has doubt in mind . He/She can call to the Contact# provided in reference letters.
Lot of Factors apart from above can trigger the call like Not Renowned Company , Low Employee Strength , Company Location on Google Maps etc.
The HR might not know your duties but he/she should know the Designation , Role , Salary and how long you have been working for the company .
Hope it clarifies your query.