There was an interesting story at Reg Hardware about cellco T-Mobile in the UK, and their response to VoIP.   T-Mobile’s Web ‘n’ Walk is a data service aimed at business people, with a flat-topped monthly tariff, however they do not want you to use VoIP or IM with this service, and it is explicitly forbidden.  To quote from T-Mobile’s own webpage:

Use of Voice over Internet Protocol and Messaging over Internet Protocol is prohibited by T-Mobile. If use of either or both of these services is detected T-Mobile may terminate all contracts with the customer and disconnect any SIM cards and/or web ‘n’ walk cards from the T-Mobile network. 

Of course this brings many questions to mind, including “why?”; presumably so that VoIP use does not threaten the normal call revenue.  Another important question is “how?”, since much business traffic is secured by VPN and so it would be impossible for T-Mobile to tell email from VoIP, IM or anything else.  Researchers have documented that the Skype client uses random TCP port numbers, and that the line protocol has been deliberately  obfuscated in order to conceal how it works.  In short detection of Skype traffic is not trivial.

All in all it’s a very interesting example of how the collision of Internet and mobile technology is causing discomfort to telcos.