place a monthly cap on my SIP trunk bill, for a number of reasons, but primarily to limit my financial risk in the event of a hack, without having to monitor it carefully.

I strongly support this suggestion. The reason why I have not availed myself of AA's VoIP service is the uncapped risk if someone successfully impersonates me. While I am not completely clueless when it comes to IT and security, I know that there are many people who are more skilled than I am who are strongly incentivised to find ways of breaking security, especially when it comes to making phone calls. Putting a configurable hard cap on the monthly bill would mean I'd sign up. As it is, I won't.