The channel variables for this purpose seems to be
{duration, billsec, answersec, flow_billsec, mduration, billmsec, answermsec, uduration, billusec, answerusec}
see: source code "switch_channel.c";
and wiki document on categories: channel variables
see: http://wiki.freeswitch.com/index.php?title=Category:Variable&from=Variable+billmsec 
 
However, when I tried to access these channel variables from Lua script or dialplan, their values are always "nil" or empty.
 
-- dialplan:
{
   <action application="answer"/>
    ........
   <action application="log" data="INFO billsec: ${billsec}; billmsec: ${billmsec}"/> 
   <action application="sleep" data="2000"/>  
   <action application="hangup"/>  
}
 
-- Lua script:
2011-12-10 13:39:55.609375 [ERR] mod_lua.cpp:191 C:\c4dev\freeswitch\Release\scr
ipts/test1.lua:60: attempt to concatenate global 'ctsec' (a nil value)
stack traceback:
        C:\c4dev\freeswitch\Release\scripts/test1.lua:60: in main chunk
 
see:
{
session:answer()
.........
ctsec = session:getVariable('billsec')
ctmsec = session:getVariable('billmsec')
freeswitch.consoleLog("INFO","***** Call-Time: sec=" .. ctsec .. "\n")
freeswitch.consoleLog("INFO","***** Call-Time: msec=" .. ctmsec .. "\n")
session:hangup()
}
 

From: curriegrad2004 <curriegrad2004@gmail.com>
To: FreeSWITCH Users Help <freeswitch-users@lists.freeswitch.org>
Sent: Wednesday, November 30, 2011 11:15 PM
Subject: Re: [Freeswitch-users] Question Answertime variable?

billseconds or bill_msec would be the one to turn to.

On Wed, Nov 30, 2011 at 4:22 AM, Thomas Hoellriegel <admin@blindi.net> wrote:
> Hi all,
> Is there a possibility to determine the caller time for a channel at
> the end of a call?
>
> For example: in asterisk exists a variable:
> ${ANSWEREDTIME}
> I like to store the Answertime in a Database for example:
> Dailly minutes to call a Cellphone.
> Can your help plese?
> Thanks.
>
>
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