some more VOIP info found in the internet
This is not a recommendation, make your own judgement on each link:
Two way VoIP over wireless nets requires low latency
http://www.globalipsound.com/customers_partners/pdf/iApplianceWeb.031201.pdf
How does Prepaid Calling Card Operation work?
http://www.qiiq.com/products/productsQPPCC.htm
VoIP.org.uk - the UK resource for IP Telephony