Hi, I'm a system engineer, activaly working in a country-wide ISP. Besides Linux based services, I aslso deal with PBX solutions, namely Asterisk.
I would suggest a CentOS based dedicated server with Asterisk/FreePBX VoIP server running on it. It's because FreePBX is easy/simple to administer by you (add/remove SIP extensions etc) after (or if) I'm no longer maintaining it.
About the card calling service. Do you have SIP access to it? If not, the VoIP server will have to have some additional interface to the land line (ATA) or mobile line (GSM gateway) to dial out to the card calling service.
If no SIP access, I can also suggest to change your calling provider to one that has SIP access so you don't need aditional hardware to the VoIP server and the implementation is more safe and simple.
My Skype ID is clouseau_jacques
Best Regards,
Istvan Prosinger