IAX2 trunk authentication / "No authority found"

I am running Asterisk 11.6 on a server and 11.2 on several routers that connect to each other via iax protocol.

Server:
store-template
type=friend
secret=ASECRET
auth=plaintext
qualify=yes
context=fromiax
peercontext=fromiax

0001
host=XX.XX.XX.XX

0002
host=XX.XX.XX.XX

0003
host=XX.XX.XX.XX


Router IAX2 (store 0002)

voipservers
type=friend
username=0002
secret=ASECRET
auth=plaintext
disallow=all
allow=g729
context=fromiax
context=setmwi
peercontext=fromiax
qualify=yes
callerid=asreceived
hidecallerid=no

primary
host=primaryvoip01

secondary
host=secondaryvoip01

atlvm
host=atlvm01

And this all seems to work between server and router, and router and server.
but when I try to reach it from the voicemail server, I get "No authority found"
IAX2 for voicemail

store-template
type=friend
secret=ASECRET
auth=plaintext
qualify=yes
context=setmwi
peercontext=setmwi

; Individual Stores

0001
host=XX.XX.XX.XX

0002
host=XX.XX.XX.XX

However, when I remove the “username=0002” from the IAX2 trunk on the router, then the voicemail server connects fine, as well as all other processes into/out of the router seem fine to.

Since there isn’t a username coming FROM the main server or the voicemail server, why does the username in the trunk effect this? My understanding is that username would be used for outbound connections to the primary server. But with “username=0002” outbound calls to the primary server seem to function just fine as well.

What am I missing here?