IAX2 does not hang up

[caspar]

I have 2x servers:
1) Mercury (10.0.0.161) runs Asterisk + VICIDIAL + MySQL + Apache
2) Beta (10.0.0.149) runs Asterisk only

I have some SIP phones that registers on Mercury which works fine.
I have some SIP phones that registers on Beta that is connected through an IAX2 trunk to Mercury, which seems to work fine except that it will not hang up.

A typical phone configuration looks like this in the VICIDIAL Admin page:

Phone extension: mercury:test@10.0.0.149/1000@default
Dialplan Number: 1000
...
Server IP: 10.0.0.149 - Beta
...
Status: Active
Active Account: Y
...
Client Protocol: IAX2
...
Manager Login: cron
Manager Secret: 1234
...
Exten Context: default
...

The Beta server configuration looks like this in the VICIDIAL Admin page:

...
Server IP Address: 10.0.0.149
Active: Y
...
Telnet Host: 10.0.0.149
Telnet Port: 5038
Manager User: cron
Manager Secret: 1234
Manager Update User: updatecron
Manager Listen User: listencron
Manager Send User: sendcron
...
Default Context: default

Does my configuration look okay? When a Beta-agent login to VICIDIAL, everything seems to work fine except that when the agent logout, VICIDIAL does not hang up the line. Any ideas how I could fix this?