T-Mobile spokesperson Alexander von Schmettow told the German online site, the Local that it violates the terms of agreement between customers and the company and that the "high level of traffic would hinder our network performance," and "if the Skype program didn't work properly, customers would make us responsible for it."
Skype has responded to T-Mobile saying,
I find it quite telling that Deutsche Telekom would be so bold as to announce this arbitrary blocking of Skype. They pretend that their action has to do with technical concerns: this is baseless. Skype works perfectly well on iPhone, as hundreds of thousands of people globally can already readily attest. But their announcement also demonstrates that some operators do not fear the customer or regulatory consequences of their bad behaviour. Its worth noting that even if German consumers wanted to change mobile providers, they could not: like Deutsche Telekom, every other German mobile operator contractually forbids consumers from using VoIP applications. (this is the same in France, actually).
It is astonishing that German people and legislators are allowing an Internet Service Provider to selectively allow and decline applications that users are permitted to run. Skype notes that the situation is about to get worse new EU legislation for telecoms which the European Parliament and European governments are supposed to adopt later this month legitimizing restrictions put in place by operators to users' Internet access, as long as they inform consumers.