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will NOT work as the dial application does not finish and move onto
the next priority once the call is answered.  I tried this and
confirmed my thoughts.

exten => x,1,Dial(SIP/XXXX,20)
exten => x,n,SendDTMF(1234)

The asterisk book describes the D flag in the Dial application:

D([ called ][: calling ])
    Sends DTMF digits after the call has been answered, but before the
call is bridged.
    The called parameter is passed to the called party, and the
calling parameter is
    passed to the calling party. Either parameter may be used individually.

Trying this in a dialplan:

Doing exten => _NXXNXXXXXX,1,Dial(SIP/CARRIERNAME/1${EXTEN},20,D(123))

results that you only hear 1 DTMF digit once the call is answered -
The asterisk CLI shows that 3 were send - I am assuming that the
difference is the time it takes for the audio to start streaming.

However, I was expecting to hear nothing, as the description in the
book above states that the DTMF is passed before the channels are
bridged (I understand this is around half a second after the call
signalling is answered)

Does anyone know of a way to wait x number of seconds, then send the
DTMF digits as audio once the call is answered.

Many thanks

Robert