A particularly nasty thing that some telcos/ISPs have been doing around the world is blocking VoIP traffic. While this can be left up to their discretion assuming it's within their rights, many companies block VoIP traffic for the sole reason of being able to sell VoIP themselves or because they are in partnerships with other companies offering it. In essence, they are denying service. This, of course, makes other companies mad. In the UK, mobile VoIP company Mobiboo is angry, stating that T-Mobile and 3 have blocked calls to anyone using their service, due to the high fees imposed by BT, one of the UKs biggest Telcos.

In a statement on the firm's website, the mobile VoIP outfit says: "Mobile operators 3 and T-Mobile have barred their customers from calling Mobiboo 07911 phone numbers. The companies say this is because of the high charges that BT is making to them to reach Mobiboo, although we believe that this is part of the ongoing anti-VoIP campaign."
While anti-VoIP may not be the right term, it's well known that telcos are doing this. Should they be allowed to do this? Do they have to allow any traffic over their networks? Likely, with most issues surrounding this, it'll end up in the courts.