hello,
one of the most common things always happened in voip softswitch industry – people had issues with billing disputes with each other – there are many reason for those disputes and 95% time the errors are human errors – software normally not make much issues always – i am now sharing one of the issue here which sometime happen
conversation time is lower but charged duration is high ! Please see the screenshot to understand

Now what was the error here ! Its a simple issues – when you gave rate in rate management – you sinply forget to change the pulse 6 to 1 – which means from 1-6 second whatever the duration is the softswitch charging for 6 seconds !
see the screenshot below to understand:

I hope you understand the problem and solution now easily. There are more things which also make problems for billing disputes but all can be solveable as i said mostly this happen for human error!
whatsapp me : +8801911119966 to know more
thanks