asterisk users - Mar 2011 - DAHDI, IAX2 and SIP considerations for Early-Media / Alerting

Hi,

Short version:

Is it possible or even legal to convert an IAX2 PROGRESS/EARLY-MEDIA
indication into a DAHDI/q.931 ALERTING signal when your ISDN provider
does not pass early media on receipt of an PROGRESS(8) indication?

Long version:

I have an Asterisk 1.6.2.18-rc1 based system with a DAHDI trunk (UK E1
line), also, the system has IAX2 trunks, and several SIP handsets.

All 3 protocols (q.931/IAX2/SIP) have a mechanism to indicate either
ALERTING/RINGING, or to specify PROGRESS/EARLY-MEDIA. Based on this
you'd think call setup would all work happily all of the time :) What
happens based on the call direction is as follows:

SIP -> DAHDI    ISDN returns ALERTING, SIP uses 180 Ringing, all OK
SIP -> IAX2     IAX2 returns PROGRESS, SIP uses 183 Progress, early
audio works OK
IAX2 -> DAHDI   ISDN returns ALERTING, IAX2 uses RINGING, all OK
IAX2 -> SIP     SIP returns 180 ringing, IAX2 uses RINGING, all OK
DAHDI -> SIP    SIP returns 180 ringing, ISDN uses ALERTING, all OK
DAHDI -> IAX2   IAX2 returns PROGRESS, ISDN uses PROGRESS(8), but the
caller hears no ringing.

I believe that my issue is that my UK ISDN provider does not accept
early media, and will simply send silence instead of using the
provided early audio stream. DAHDI is configured with:
? ? ? ?priindication=outofband
The IAX2 trunk provider is using early-media to send the ringing tone,
and as above, this mostly seems to work okay. The exception is when
the call is bridged to ISDN, where I believe the ISDN provider does
not pass on early media.

I checked the IAX2 RFCs 5456/5457, but cannot find a definition of how
RINGING/PROGRESS is meant to work. Is my IAX2 trunk provider doing
something wring by not also sending RINGING?

Is there a workaround that converts either IAX2 PROGRESS into RINGING,
or allows DAHDI to send ALERTING if it receives an early media
indication? I suspect the code to do the latter would be reasonably
simple, but would appreciate pointers for any badness that it may
cause.

Thanks in advance for any suggestions.

Regards,
Steve