Hello Regina,
As for billing the quintum does a good job outputing the information to the billing platform. There are many companies that sell billing solutions for Quintum. Using CDR output or the Radius IVR capabilities for calling card setups. As for echoing and call quality there is a reason that quintum is the number 2 VOIP company. Cisco is number one for sure at this point but.. Quintum is cost effiective, and a quality solution. If you are having problems with poor call quality or lag times, I would look at your bandwidth and what codecs you are using and the system software version (display v)in the quintum current version is P4-2-20-20. Codecs can be changed in the DSP area in the CLI. For prefix to carrier question it sounds like you are doing VOIP hop off calls to the PSTN? If this is the case I would take a look at lampattern and lamreplace in the pstntg in the manual. I hope this help you out.