I would suggest that if you cannot see in real time how many staff that you have, that your ACD/phine system isnt up to much.
To be able to tell how many staff you need,
a) identify the quantity of calls you receive per half hour. historical data should provide you with this, try averaging this over time and look for seasonal and individual day changes.
b) identify the call handle time, this may vary during the day as people get tired or the type of transaction vary. Handle time is talk time+wrap time+hold time. Hold time is the time a caller spends on hold when the agent is referring or looking up details and presses the hold button. hold time often isnt included in talk time. If an inbound call generates an outbound call include this time as well if not picked up in wrap time.
c) Identify what level of service you wish to provide, to do this identify how important each call is to you, if each call generates $10 and staff cost $5 an hour, you ideally want a very high service standard and vice versa.
d) then use an erlang calculator to work on the resources you need per half.
e) then schedule your staff.
f) you may find that part time staff help you to deal with the peak and troughs in call volumes over the day better than full time staff will.