1 extension returns dialstatus busy rather than dialstatus noanswer

I have one extension that after the phone rings for 3 or 4 times, it returns DIALSTATUS=BUSY and the voicemail plays the busy message. What it is supposed to do is return DIALSTATUS=NOANSWER. If I create a new extension and use the same phone it works fine -- so it is not a phone problem signaling wrong. If I delete the offending extension and re-add it, it still has the same wrong behavior. What is the problem? How can I fix it?

All other extensions work fine.

Thank you -- see below for relevant log entries.

This is the bad extension (4209):
-- Executing Dial("Zap/18-1", "SIP/4209|15|trwW") in new stack
-- Called 4209I>
-- SIP/4209-0a07d0c8 is ringing
-- Playing 'conf-hasjoin' (language 'en')
== Parsing '/etc/asterisk/manager.conf': Found
== Parsing '/etc/asterisk/manager_additional.conf': Found
== Parsing '/etc/asterisk/manager_custom.conf': Found
== Manager 'phpagi' logged on from 127.0.0.1
== Manager 'phpagi' logged off from 127.0.0.1
-- Nobody picked up in 15000 ms
-- Executing Set("Zap/18-1", "DIALSTATUS=BUSY") in new stack
-- Executing GosubIf("Zap/18-1", "0?BUSY|1") in new stack
-- Executing GotoIf("Zap/18-1", "0?exit") in new stack
-- Executing Set("Zap/18-1", "SV_DIALSTATUS=BUSY") in new stack

This is the good extension (4211):
-- Executing Dial("Zap/20-1", "SIP/4211|22|trwWM(auto-blkvm)") in new stack
-- Called 4211
-- SIP/4211-0a07d0c8 is ringing
== Parsing '/etc/asterisk/manager.conf': Found
== Parsing '/etc/asterisk/manager_additional.conf': Found
== Parsing '/etc/asterisk/manager_custom.conf': Found
== Manager 'phpagi' logged on from 127.0.0.1
== Manager 'phpagi' logged off from 127.0.0.1
== Parsing '/etc/asterisk/manager.conf': Found
== Parsing '/etc/asterisk/manager_additional.conf': Found
== Parsing '/etc/asterisk/manager_custom.conf': Found
== Manager 'phpagi' logged on from 127.0.0.1
== Manager 'phpagi' logged off from 127.0.0.1
-- Nobody picked up in 22000 ms
-- Executing Set("Zap/20-1", "DIALSTATUS=NOANSWER") in new stack
-- Executing GosubIf("Zap/20-1", "0?NOANSWER|1") in new stack
-- Executing Goto("Zap/20-1", "nextstep") in new stack
-- Goto (from-did-direct,4211,21)