I realize forgot to mention,
all my a2billing solution based on realtime sip/iax friends,so when ucreate customers , u dont need generate a2billing_additional_sip.conf, a2billing_additional_iax.conf,
this is useful for large business, because if you have>1000 users, the conf file is large size,it will make your system slow
another advantage is : when you have customer auto signup function,after one customer is created automatically, he can register to the system immediately.