extensions.conf:
[fail-test] ; to minimize the # of things in the call, test using the console channel driver: "channel originate Local/s@fail-test application Wait 35"
exten => s,1,Answer()
    same => n,MixMonitor(fail-${EPOCH}.wav)
    same => n,Originate(Local/s@bridge,exten,outgoing-ring,2579,1,,a)
    same => n,Goto(bridge,s,1)

[bridge]
exten => s,1,Answer()
    same => n,ConfBridge(testbridge,silentbridge,incogtalker)

[outgoing-ring]
exten => 2579,1,Set(CDR_PROP(disable)=1)
    same => n,Answer()
    same => n,Dial(Local/${EXTEN}@originate-local/n,,G(split))
    same => n(split),Hangup()
    same => n(callee),Goto(bridge,s,1)

[originate-local] ; actual outgoing call
exten => 2579,1,Dial(IAX2/REDACTED/REDACTED) ; <-- this is a call to another Asterisk which uses DAHDI to complete a call to an analog switch using an analog trunk. On its end, it uses the D option with Dial(DAHDI...) and if DTMF is sent on answer, that's when all this weirdness happens!
    same => n,Hangup()

confbridge.conf:
; these are provided mainly so that the confbridge is silent and not in our way, so we can hear everything that's happening, all the time
[silentbridge]
type=bridge
sound_join=/var/lib/asterisk/sounds/en/silence/1
sound_leave=/var/lib/asterisk/sounds/en/silence/1

[incogtalker]
type=user
dsp_drop_silence=no
quiet=yes
dtmf_passthrough=yes
answer_channel=no