asterisk users - Apr 2006 - External voicemail and MWI on internal phone

Hi,

I have a Cisco 7941 phone running SIP, and for a variety of reasons [1] have
configured this in the short term to use my local Asterisk server to register
to, and then have my local Asterisk server in turn register to the upstream SIP
proxy at my voice provider.

Voicemail is supplied by my VoIP provider, so I've no real need to set up
voicemail locally.

My provider also runs Asterisk, and correctly sends me down these headers:

Messages-Waiting: yes
Voice-Message: 1/0 (0/0)

However the local phones never see the headers and thus never know that there is
voicemail waiting upstream.

Is there a way to forward on or regenerate the Messages-Waiting and
Voice-Message headers from upstream, to each and every local phone in the same 
context so
that any and all phones on the local network see the voicemail present and have
their MWI light up?

Thanks,
Reuben


[1] So far I've been unable to get the 7941 to do authenticate to my
provider, unsure quite why, the phone seems to just ignore WWW-Authenticate
requests from upstream.  A Cisco 7940 and ATA-188 work fine so it may be a bug 
or a feature that needs to be configured - dunno.