This patch will allow subscribing to the message waiting indication (MWI) on a remote asterisk server.
It works on a push model. When a message is waiting on one asterisk server, it will inform the asterisk client.

SIP client --- SIP ---> asterisk server 1 --- IAX --> asterisk server2

To inform the SIP client (see above drawing) that a message is waiting on asterisk server2, configure sip.conf on asterisk server1 as follow:

[sipclient]
type=friend
mailbox=IAX2/iax_channel_name/mailbox_number

So if you want to check the mailbox number 500 on asterisk server 2, connected via the IAX2 account "hydrix" on asterisk server 1 ; you will put in sip.conf:
mailbox=IAX2/hydrix/500