Outbound MWI subscriptions

Any tips on making this work? Server 1 behind a NAT router, server 2 on the internet with static public IP.

Connected with via IAX. I want a phone on server 1 to share the voicemail of a phone on server 2.

On server one, I have a line:

mwi => 2002:my_safe_password@servername.com/2002

After applying this, I can see this:
CLI>sip show mwi
Host Username Mailbox Subscribed
servername.com:5060 2002 2002 No

I then changed the line for the phone on server 1 (dahdi - maybe that is the problem?) and it seems to be fine as well:

CLI> dahdi show channel 4
(snip)
Mailbox: 2002@SIP_Remote
(snip)

Why does subscribed say “No”?

Am I missing something? I am having trouble finding information on this feature other than what is in sip.conf.sample.

I have not looked at wireshark captures yet.

Thanks!

edit is SIP_Remote correct - just as typed? Or is that supposed to be something different.