I am also not at all an expert in GPL licencing or IP, but would like to add that any customisation you make specifically for the purpose of interacting with a billing server or other external application is likely to take the form of a Kamailio module, rather than modifications to the existing Kamailio modules, the Kamailio core, or any other part of the stock code tree.
As far as I know, under the GPL--broadly speaking, for I certainly don't know or remember the specifics--modules get different treatment than modifications per se.
As far sa I know, if you can stick your code into [a] completely self-contained module(s) (and given what you've said about what specifically you want to accomplish, it seems to me that you absolutely can) that interact(s) with Kamailio using its module APIs and requires no modifications to Kamailio itself, it's not subject to the same licence constraints as modifications per se.
Please do not take my advice as stated, and consult a competent software/IP attorney.
-- Alex