[Asterisk-Users] asterisk t.38 codec negotiation problems

[bladerunner]

hello list (3rd try as my first post seems to have gone astray in the endless 
realms of tcp/ip and in my second i accidentially replied to another post not 
related to the problem),

i searched for nearly a week for a solution to this problem, as there is:

analog fax machine -» grandstream ata -» asterisk -» sip trunk from provider 
-» provider gateway to pstn -» analog/isdn fax machine on pstn

everything worked out fine until my provider decided to implement t38 into the 
gateway. now when i send/receive a fax message the gateway tries to connect 
with t38 and waits for a "415 unsupported media type" to receive if the device 
on the sip trunk is incapable of talking t38, in order to fall back to the 
old g711 behaviour.

but asterisk does not answer with "415 unsupported media type", he answers 
with his own codec capabilities, 
and this is ignored by the gateway. so he tries to send with t38 anyway, and 
that of course fails.

call flow:

should be: reject t.38, fallback to g.711

PBX                 GW
 |                   |
 |    INVITE F1      |
 |------------------>|
 |  (100 Trying F2)  |
 |<------------------|
 |   180 Ringing F3  |
 |<------------------|
 |     200 OK F4     |
 |<------------------|
 |      ACK F5       |
 |------------------>|
 |       G.711       |
 |<=================>|
 |    INVITE F6      |
 |<------------------|
 |  (100 Trying F7)  |
 |------------------>|
 | 415 Unsupported F8|
 |------------------>|
 |    INVITE F9      |
 |<------------------|
 | (100 Trying F10)  |
 |------------------>|
 |     200 OK F11    |
 |------------------>|
 |      ACK F12      |
 |<------------------|
 |       G.711       |
 |<=================>|
 |                   |

F1:  m=Audio, PCMA
F4:  m=Audio, PCMA
F6:  m=Image, t38
F9:  m=Audio, PCMA
F11: m=Audio, PCMA


is: acknowledge with g.711 on t.38, call rejected by gateway

PBX                 GW
 |                   |
 |    INVITE F1      |
 |------------------>|
 |  (100 Trying F2)  |
 |<------------------|
 |   180 Ringing F3  |
 |<------------------|
 |     200 OK F4     |
 |<------------------|
 |      ACK F5       |
 |------------------>|
 |       G.711       |
 |<=================>|
 |    INVITE F6      |
 |<------------------|
 |  (100 Trying F7)  |
 |------------------>|
 |     200 OK F8     |
 |------------------>|
 |                   |

F1:  m=Audio, PCMA
F4:  m=Audio, PCMA
F6:  m=Image, t38
F8:  m=Audio, PCMA

any thougts on that one?

kind regards,

michael