Hmm, not really. My experience of phone systems unfortunately is the other end of them. If that's the case, I'm assuming that the time is recorded as minutes then? (e.g. 60 for lunch, 15 for break?) If that's the case, then it should be just a case of doing 2 select SUM statements, adding those and subtracting 75 from them.
If they are start and end times (2 different fields) then that would be more difficult. Maybe someone who admins the Avaya system might have a better clue than I would.
Are you performing the queries in SQL by the way?