Ella
It could be either a Work Focussed Interview or a Compliance check, I am afraid without more information it's impossible to tell which it is.
If it is a WFI then I would have expected your relative to have had previous contact with their local JC+ office, it is very unlikely, if this is their first WFI, that it would be done over the phone unless it was pre-arranged.
Work Focussed Interviews If it is a compliance interview then there may be questions about the benefits your relative gets and their income and savings, more often than not, a compliance interview over the phone is done because the DWP think that the claimant may be entitled to extra benefits, perhaps a Premium is payable.
Gordon