Are there any VOIP providers that provide $ limits with their plans.  I've done a bit of reading on asterisk security, and it seems that monthly exploits are published, and the final defence should be a $ limit to prevent loss of too many $$.

2talk has I think one in that if you don't choose to auto topup ( and who would ), your losses are limited to whatever credit you have with them.

I don't know if WXC has one though I did ask once about toll blocking on a VFX phone, and I was advised that it costs $10/month extra as they lose the toll income. 

Ward Mundy on the PIAF forum keeps recommending the DLink wbr-2310 as a cheap firewall as well so if I can not get a $ limit I will be looking at hardware firewalls.