Hi
Bear in mind that the code base of A2Billing is about 10 years old, and if starting with a blank sheet of paper with today's technology, other decisions may have been made. However, a2billing.php and it's methodologies have served us well over the years.
We have companies using A2Billing with tens of thousands of customers without issue, some of them distributed systems, e.g. DB and Asterisk server(s) on separate hardware, and this is a method we would recommend to increase capacity.
For raw capacity in a wholesale environment, although I have not tested to capacity, simply that it works, you could consider
viewtopic.php?f=18&t=10428Joe