[Serusers] how does a b2bua help billing

[Iqbal]

Hi

I am curious as to how a b2bua does the billing side of things, I know it
sits in the middle and the media goes through it, but how does it keep
count of how long the session has been open for, does it keep a
continously running count of time after a INVITE or use some other
non-evasive method.

If it keeps a continous count (I presume in memory), how does is initiate
a disconnect, does it compare the time used with that left (which is
determined at call start)

Any links , pointers,

Iqbal