> Just looking for comments

I've worked with a bunch of different implementations over the years, in
telephony apps.

It's all about the compromises.

You can get a nice DTMF encoder from Holtek for $0.25 in relatively
small quantities.  That's a lot of codespace and processor time freed
up, and the chip is easy to talk to.   

Decoders are a bit more, but I think you'd be hard pressed to get equal
performance in all respects, from a software solution, especially if you
are looking to do anything else in the same processor. Making something
that sort of works is one thing. Making something that works with good
dynamic range, twist tolerant, capable of detecting in 40mS, tolerant of
noise.. That is likely to involve significant development time if it can
be done at all.





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