Vodafone's app for corporations allows them to make secure calls
Vodafone's encryption app would seem to be quite secure
While Vodafone is extremely confident with the 128 bit key encryption, Deutsche Telekom has a similar app available that launched last summer. But Deutsche Telekom's app uses an AES 256 bit key which cannot be cracked by any currently produced computer. That exceeds the standards set by the German Federal Office for Information Security.
The Deutsche Telekom app includes an option for encrypted texts, a feature that Vodafone says that it will add to its app. Both will work even when SIM cards from other carriers are employed, and both will encrypt calls over Wi-Fi. The carrier charges the equivalent of $16.90 USD monthly for a single line license. Deutsche Telekom's encryption app costs between $17 USD and $24 USD a month.
source: WSJ, EETimes