With this filter curve or the filter curve generator for the filter curve equalizer the sound can be filtered in such a way that the DTMF tones are highlighted to be able to decode them more easily. Different EQ curve presets are available for download.
I use https://play.google.com/store/apps/details?id=com.audiocommunication This App gave the best DTMF tone decodings results compared with a few other free apps years ago. If you know better apps, please share, especially because this app is so ad-infested. With this app it is possible to decode DTMF sequences when there is only few background noise. When there is background noise, then this project into play.
Use the DTMF Filter Curve_2Hz.txt directly and import it in Filter Curve EQ of Audacity. Paramters for generation were bandwidth 2 Hz, baseline -30 dB and boost 5 dB. With these settings I was sucessfully able to decode the dialed sequence at the jingle of "Bei Anruf Erfolg" from Umberto Saxer, it is 15097.
There is another DTMF Filter Curve_5Hz.txt with the same parameters, but 5 Hz bandwidth.
A bandwidth of only 2 Hz and even 5 Hz is very small but was necessary to decode the DTMF sequence of "Bei Anruf Erfolg". There is music around the sequence and with a larger bandwidth the frequencies of the music are decoded as DTMF tones. So for decoding in your case a larger bandwidth might be needed.
Relevant parts to change
####### Change here ##########
bandwith=15
baseline=-30
boost=5
####### End change here ######
baseline is the attenuation of none DTMF frequencies and boost the amplification of DTMF frequencies.
Copy the output of the script to a text file and import it in Audacity. For this project Audacity 3.1.3 was used.