[robertlagrant]

I would love a way to make an IP phone that just lets my kids call their friends on their phones, and no-one else. I will take a look!

[otterpro]

see TinCan phone mentioned in the comments, which does this. However, it means that all their friends need to buy the phone as well.

[robertlagrant]

Ah - wonderful. I didn't know what that was. Sadly it's only US and Canada, but very cool to see someone's thinking about this.