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Well maybe their jobs could be saved by dedicating them to reliable tone production over TCP/IP. Ever try to access a mailbox or navigate a customer support call center through Skype?

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How can the fact it's TCP/IP interfere with the tones? TCP/IP only sees bytes.

Audio compression optimized for psychoacoustics, I figure.

I doubt you'd be able to plug a dial-up modem into skype and achieve a working computer connection. Pure tones (as from key presses) is a less extreme case, but it's a conceivable problem.

To add to your point: Skype uses SILK codec, on which OPUS, the currently preferred codec for internet-audio, was based.

If you do an outbound call (i.e. outside Skype, to something that would actually use DTMF) Skype doesn't use SILK/Opus on the outbound connection. You can tell because the quality is way worse.

There are also issues with the echo cancellation Skype uses. If you have issues with DTMF, that's the likely cause, not the audio codec.

But echo cancellation should not be a problem on mobile devices.

Disable your echo cancellation. It breaks DTMF.

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