Here is a suggestion
1. Assume 8K per call center staff. for 120 seats,this will work out to 960 K.
2. Assume that you have 10% additional staff in supervisory cadre (or 12 seats) who will use 40% of time. This requirement will work out to 38.4 K
3. Assume overhead of 20% to both the above. The total will be 1.2*998.4 = 1198.08 KB.
4.Take a bandwidth of 1280 KB. you can provide additional provisions for growth and other traffic
this link could be interesting
Hope this helps