I have attached a spreadsheet to hopefully make this explanation easier. I have two Sheets ('SLA's' & 'Calculations').
In 'SLA's' this shows the 'Start' and 'Finish' time for each priority (I.e. P1's and 2's continue through the day/night, but P3's start at 08:00 and stop at 18:00) and shows their SLA's (2 hours, 4 hours and 8 hours).
In the 'Calculations' sheet; what I would like to do is, based on the priority, calculate when the call will fail. So, for example, if a Priority 1 is raised on 21/09/2012 10:00, and it's SLA is 02:00 hours, it would fail at 21/09/2012 12:00. If a Priority 3 is raised on 21/09/2012 17:00, and it's SLA is 08:00 hours, it would fail on 22/09/2012 14:00.