[Asterisk-Users] Strange DTMF issue with callback

[Asterisk Manx]

Hi 

I'm trying to implement a Callback mechanism whereby I generate a Call
file and connect an arbitrary extension with my cellphone (via a SIP
Channel).

If I create a .Call file that connects the channel
"SIP/12345678 at Provider.net" with a local extension/context I get some
weird issues with DTMF tones.

I've set dtmf=2833 and the codec in use is G711a. 

For example - I create a .Call file that calls my Cellphone (12345678)
and connects it with the local extension "s", priority 1 in context
"CallBack"

[CallBack]
exten => s, 1, Answer
exten => s, 2, DigitTimeout(5)
exten => s, 3, ResponseTimeout(10)
exten => s, 4, SendDTMF(1234)
exten => s, 5, DISA(no-password|CallBack)

exten => 99,1,Playback(demo-thanks)

My cellphone rings, I answer and hear the DTMF tones sent by Asterisk.
The problem is Asterisk is unable to recognize any DTMF tones I enter
on my mobile, thus I'm unable to do anything with the dialtone (hangs
up after 10 seconds).

However - if I change the [CallBack] context to;

[CallBack]
exten => s, 1, Answer
exten => s, 2, Dial(SIP/MyPhone)

My local SIP phone rings and I'm then able to hear (and transmit) DTMF
tones from both end points. As a sidenote, I can make calls in and out
the provider and DTMF works fine.

Does anybody have any ideas - any help is appreciated..

Cheers

Joe