I’m running a small call centre here in Australia - here are my specs:
2.8 GHz Pentium 4
1 GB RAM
40GB Hard Drive
30GB Download limit per month.
I have 12 people in the call centre, with around 4 on call at any time 7 hours a day.
I am using G729 Codec and my usage per month is 8 GB (download and upload combined).
CPU usage is hovering around 6%, and RAM usage is at 300 MB (Asterisk is 20MB from that).
It is an outbound only call centre and the server is on our premises, so I am not counting the data between the server and the agents, only the server and the VoIP provider.
With the specs you got, and, provided that the link is fast (i.e. > 1.5Mbit/sec) and has low latency (<200ms), you should be able to support around 20-30 simultanious calls - using G729 - which I would recommend to minimise usage and bandwidth.