Just to clarify, vpn would allow direct, encrypted sip calls to be made *without* using any VoIP proxy (so-called providers), just direct communication. It is what you meant anyway, probably, but some may mix it up with next sentence about "finding provider, that don't bends to the feds".
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The interesting idea from this topic, is to use *plain* voice call for encrypted data transfer (at low bandwidth, using tones). This is something I would *really* like to see. Not that it would be replacement for every-day usage, but in case of some problems (like data transfer getting emergency shut down), would be very practical.

Of course, it still require device able to decrypt and encrypt on both sides - but, this time, one of such device could be plain pstn telephone, forwarding received sound to computer (in case of another side lacking N900). Either plain as in "plain", with speaker put to computer microphone, or - more reliable, 5$ PSTN phone modified (super-easy modification) to pass sound into computer line-in, instead of built-in speaker (jack audio output).

Heck, same project - due to code openess - could be used even *without* any N900's, just using computers and mofified PSTN phones.

/Estel

/Edit

I'm pretty sure, that there are some FOSS projects on tonal data transfer already, that could get harvested.