We are starting to set warning emails based on VoIP usage.
On the control pages, a VoIP number has a 'bill warning' amount set. When this amount is reached we'll email the email on the number and on the login it's associated to.
The default for new numbers is £10, for number who spent more than £10 in September we've set the level to 1.2 times Septembers level.
For more information on VoIP security please see this wiki page:
http://wiki.aaisp.net.uk/index.php/VoIP_Security
This feature does not cut the service off.