Asterisk Billing

hi i am a newbie in Asterisk.Can any one please suggest which is the best billing software i can integrate .As my understanding a2billing is some thing like we can create an account for each user and need to credit the account.what i am looking is just at the end of month i need to billing for each extensions.is this possible in A2 billing.
i know with elastix there is a billing option.
which is better for my requirement?