Calculating time difference between the same field of different records

[mujaqo]

Please help,
How can I calculate the time difference between the current record time and the previous record time for the same phone number as shown below?

Phone Number	Call Time	Reason	Time diffrence between this call and last call for the same phone number
616222111	06/04/2014 10:04	Appointment	0
616222111	06/04/2014 11:58	Inquiry	62
616222111	06/04/2014 12:39	Inquiry	41
616222111	06/04/2014 13:23	Complaint	44
616222111	06/04/2014 13:24	Appointment	1
545566321	06/04/2014 15:51	Inquiry	0
545566321	06/04/2014 19:06	Inquiry	195
545566321	06/04/2014 19:36	Inquiry	30
282244589	06/04/2014 13:11	Appointment	0
282244589	06/04/2014 13:31	Complaint	20

[orange]

I'd suggest a recordset, sorted ascending on PhoneNumber and CallTime.
Read a record store the time,
read next record, calculate CallTime - stored CallTime
refresh the storedTime
Loop