Curly, Unfortunately (never could spell that word) I have seen some slowing during use, but only on a fairly large system. Recently did a small MICS with about 30 stations and 1 extra trunk module, No DTI, no voicemail. Could not tell it was running.
Same thing with NRU Backup Utility. Will slow a fully loaded system, but not see the effect on smaller system.
You might consider doing it over their lunch hour, but then again, 1 hour might not do it. Very slow indeed.
MarvO said it