This is the instruction on how to install a2billing with FreePBX:

basically, 1 extension in freepbx is 1 card in a2billing. so 1 extension in freepbx represented as a card in a2billing a card holds value of call-plan a call plan contains one or several ratecards a ratecard contains one or several rates a rate is where you configure rate per destination, by time, whether or not its progressive in a rate also configured which trunks will be used so: card –> call plan –> ratecard –> rate –> trunk to configure it, usually from bottom to top, that is from providers –> trunks –> rates –> ratecards –> callplans –> assign call plan to a card

read this Online manual

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