As I spent years working on billing for a world leading telcoms company I have to ask What do you mean by "billing application" your q implies you want to inpu

As I spent years working on billing for a world leading telcoms company I have to ask What do you mean by "billing application" your q implies you want to input transactions into some accounting (presumably double entry) system? Does not your Accounts receivable have a web frontend or an API? do you really need to build your own accounts receivable / invoicing system