=Date( CallDateTime + ( EnquiryDuration / 86400 ) , 'DD/MM/YYYY hh:mm:ss')
86400 is the number of seconds that there is in a full day when it comes to how datetime values do math.
From the online Qlik Sense help manual pages:
Date and time values are based on a date-time serial number that equals the number of days since December 30, 1899. The integer value represents the day and the fractional value represents the time of the day.
Read more here: Date and time functions ‒ Qlik Sense