Here's the text from the bottom of their site:
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.
They're blocking VoIP and messaging over IP? I can't install a simple text messaging client? Unbelievable. So basically they are forcing me to use SMS, which carries a per-message fee. Isn't that grand? It gets better. They also don't want any heavy data users on their network even if you purchased the "unlimited plan":
To ensure a high quality of service for all our customers, a fair use policy applies. T-Mobile defines fair use as total UK data use (both sent and received) of up to 2GB per month. T-Mobile may contact customers who exceed this volume of data in two (or more) consecutive months in any six month period to ask them to reduce their usage. If usage is not reduced, notice may be given, after which network protection controls may be applied which will result in a reduced speed of transmission.
I guess "unlimited" is really "limited".
T-Mobile is trying to protect their voice revenues from VoIP companies such as Mino Wireless, which I recently discovered. Mino Wireless recently launched a Java-based VoIP softphone that performs VoIP-over-mobile calls, offering rates as low as US$0.02 per minute for international calls. That's really cheap. No wonder T-Mobile wants to block VoIP. Of course, I already knew that VoIP would be cheaper than mobile international calls.
Mino's Java-based softphone works on Blackberries, Palm-based devices (Treo), Windows Mobile devices, Symbian phones, and Docomo. So if you want to take your chances on cheap VoIP mobile calls using T-Mobile's service and not getting the boot, check out Mino.
As far as T-Mobile and the VoIP and text messaging ban, my guess is that T-Mobile won't be able to sustain this business model for very long. Eventually customers will demand these applications and competition will force T-Mobile to remove the ban. Maybe we should sick the rabid Net Neutrality proponents on the cellular carriers?