Thanks, @sbiddle for the tantalising comment. While I don't mind too much the risk of guests bypassing my (rather limited) security on the outgoing calls, I would be curious to know how to go about strengthening the security a bit, since I haven't a clue how to do it.
sbiddle: You need to set up a explicit rules only allowing numbers you want. Any generic rules such as X. in an outbound route introduce loopholes.
Can you explain this further? As far as i understand this will just match anything with 00,04,07,09 and nothing else. How does it make it insecure? Is there something they can dial to circumvent this? Apart from blocking individual area codes what more can you do?