说明：  基于开源电话系统asterisk的api写的一个简单计费，计费分两部分实现，一是帐号费用的检查，另一是扣费的实现
(Asterisk open source telephone system based on the api to write a simple billing, billing to achieve in two parts, first, the cost of checking accounts, the other is the realization of chargeback)