Shalini,
No restrictions on VOIP but u cant port a PSTN line to a VOIP. For that you have to be an ISP with an ILD license
For a Call center however the qulaity of VOIP calling cards solutions on offer are really not upto the mark from what i have been using
You would need a dedicated IPLC to the geograhic region where u wish to originate calls from or make calls to