Karlsruhe Institute of Technology (KIT) has come up with a way that makes room for soundless calling, where it is on exhibition at CeBIT 2010. To sum it up in their own words, here it is below.
The Institute for Anthropomatics has developed a technology that enables people to speak soundlessly and still to be understood by the conversation partner. This technology is based on the principle of electromyography, that is the acquisition and recording of electrical potentials generated by muscle activity. This muscle activity is measured in the face. Four applications of this technology will be exhibited. An example is soundless calling. The user can speak into the phone soundlessly, but is still understood by the conversation partner on the other end of the line. As a result, it is possible to communicate in silent environments, at the cinema or theater, without disturbing others. Another field of use is the transmission of confidential information.
If you opt to transmit passwords and PINs, you are able to change seamlessly to soundless language, transferring confidential information in a tap-proof manner. Neat, eh?