ehari,
12/1 ratio doesn't leave much time for the person doing the monitoring to get other tasks done unless that's all that person is doing. Look at these numbers and see which one of these may fit for the person doing the monitoring, I just figured a % that will equal how many hours per day it would take just to monitor phone calls and nothing else. (based on a 5 day week)
3% = 2.88 hrs per day
4% = 3.84 hrs per day
5% = 4.8 hrs per day
6% = 5.76 hrs per day
Regards, Cordon