The operator justified the surcharge by explaining that billions of euros invested by operators in the roll-out of networks in recent years were based on rate costings with income from voice telephony and mobile data.
Georg Pölzl, managing director of T-Mobile Deutschland, said that if this basis is no longer certain, then neither is the operational future of the networks.
"T-Mobile wants to continue offering its customers state-of-the-art technology in future and needs a reliable basis to do so," he said.
Vodafone is expected to announce a similar policy for the use of Skype or other VoIP applications on smartphones.
Pölzl said the T-Mobile VoIP service will be available this summer.
He said pricing would be staggered and customers would be able to use the VoIP services of different providers.
"In this way, we are building a bridge between the different customers’ needs for the most competitively-priced and innovative services," he said.
Pölzl said the charges were necessary because additional investment in the network is necessary for simultaneous, "always on" use of VoIP applications".
He added: "It would not be fair to customers who don’t use VoIP if these additional costs were to be shared across all customers.
"For this reason, we are making it possible to use Internet telephony via optional rates, while keeping it otherwise barred."