As root you have access to some ring buffer logs; the default size is 4 x 16 KBytes and you may read the ring buffer with something like
$ sudo /system/bin/logcat -b radio -v time
which lists and waits for further data written to the ring; or with
$ sudo /system/bin/logcat -db radio -v time > /tmp/radio.log
to copy the actual content for 'radio' away; other -b values are: 'main', 'system', 'radio' or 'events' and there are a lot of filter flags; just run the command with -h to understand them;
Side note: DTMF data is visible in the ring for around 7 minutes; be careful after entering PIN or other sensitive data as DTMF into phone calls.
Last updated: Sat Oct 17 15:35:27 CEST 2015