The thing is that I know many applications, like Cisco 5350, SNOM SIP SERVER ( www.snom.org ), that send the Acct-Session-Time to RADIUS. In fact, RADIUS (at least mine, IC-Radius) is ready for receiving such attribute. Why should not SER send this standar attribute?. If I want my RADIUS to estimate the Acct-Session-Time, I need to touch my RADIUS' code. And touching code means not a standar way of achieving accounting, because, I can modify my code, because it is open source (IC-Radius). But what would people do if they do not use IC-Radius or FreeRadius? I do not want to modify my code ( unless there is no other way of getting the session time).
Radius was created for Dial-Up Access (Remote Authentication Dial-In User Service). You should not expect all attributes to be fully functional in a VoIP application. It was not designed for that.
Regards,
Lucas