Actualy this is question a not for A2B community, but for Asterisk community. You can ask it on forums.digium.com
If you interesting about calculate your traffic, use Google => "voip traffic calc".
Added after 4 minutes:
Also, please do not double your posts by Private Messages to me!!!
PS: To All. Do not send any questions what is NOT PERSONAL to my PM, please!! This is no good idea (for public questions we have this forum!!) and all this PMs will be always sweep with no answer and senders will be ignored. thnx for understood.