On: Microsoft to shut London Skype office putting 220 ...

[vetinari]

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

[gcp → vetinari]

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.

[orbitingpluto → gcp]

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