Actually your software DTMF code saved me from giving up. And yes DTMF CID is niche and rare but it could be repurposed and help some body.
Have you checked the errors in the schematic, as I mentioned in your cross post on this topic?
Actually no I'm very new with KiCad and a simple circuit like this should not have errors.
I agree, but it is hard to see how the published circuit could do anything useful.
Please answer the question about the intended gain and frequency response.
It's sad to hear it. 
I just published the used circuit. Also there is the software (USB/DTMF) part that can be interesting for some one.
Mystery solved. The circuit published on the Github page is NOT the same as the one published in post #18 above.
The decoupling circuit shown in #18 above (1K/100uF), which is missing from the Github schematic, provides a very small amount of negative feedback.
The op amp gain is -8 dB at 10 Hz, and drops to -44 dB at 4 kHz, so the op amp is acting to reduce the signal level (substantially at high frequencies). That probably will work for a land line, in some cases.
See simulation in the OP's cross post: DTMF Caller ID logger over USB-HID (VUSB) - #5 by jremington
hi, it seems down of removed, it would be nice to know whether any cached and available for forking.
for GitHub - kingofnull/CIDL-USB-Bridge: A DTMF Caller id logger which works as USB based HID device.
Hi @xiao_laba. Hopefully @kingofnull will be able to provide the project, but in case they don't respond it might be useful to know that an archives of the home page of the repository is available on the Internet Archive Wayback Machine:
Unfortunately the rest of the repository was not archived, so it isn't much, but maybe it will provide some useful information.
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