Thanks! was able to do this with the help from
http://forum.asterisk2billing.org/viewtopic.php?t=998&
1. In a2billing.conf under agi-conf i set sip_iax_pstn_direct_call_prefix = 0810235
2. I added rate and set prefix as 0810235
3. I created a trunk with %cardnumber% as the provider IP "%dialingnumber% doesn't work for me"
4. I created a rate card and associated to the trunk i created
5. I added the rate card to my existing Calling Plan
6. Then in the did destination i add the prefix before the extension number eg: extension number is 0001 so in destination i put 08102350001 an set VOIP_CALL to NO