first of all u would have to calculate the number of average calls and the average holdtime. By this calculation u can know what bandwidth u have to go for and what layer2 u can go.
u can go for frame-relay or a IPLC. Please remember, the more the number of hops, the worst the quality of the voice.
What seater your call center is (60 seater, 100 seater etc).
Coming to the equipments u really need a friendly softphone for your system. there are also hardphones.
next thing is we have to determine wether its an outbound or an inbound call center.
outbound call center is pretty easy with the hardphones as well as softphones.
Inbound call centers need what is known as Direct Inward Dialing (DID). This can give you toll free numbers for the customers as well as it can be economical for the call center too.
finally, the call center staff has to be educated on the delay, echo
and the call quality so as to get them adjusted to the phones.