Well, if you want to go with Prepaid calling cards (IVR) and a lot of features and options…
There are a lot of VoIP billing systems with a price from $4000 to $50 000 (the Cisco’s solution).
Good ones (I am telling this from experience) are RODOPI and VoiceMaster. VoiceMaster costs at about $10K with everything (even the Radius). In RODOPI you have to buy also the Vircom VoIP Radius server (about $3,5K).