Voicemail Records 30sec of Dialtone after Zap Disconnects

I changed POTS providers. Now my Asterisk voicemail is full of dialtone.

Holding a normal phone to the wall, after a call I hear 30 seconds of dialtone, then a sharp broken j,-j,-j,-j,-j,-j… sound (more discordant than a busy signal), then silence. I do not hear the two prominent flashclicks that I was accustomed to from the real phone company.

The voicemail mercifully cut off after 10 seconds of silence. I added the following to zapata.conf. Now it cuts off after 5 “j” tones, which is marginally better.

[channels]

callprogress=yes
busydetect=yes
hanguponpolarityswitch=yes

For a point of comparison, my slightly defective $20 AT&T POTS phone with integrated answering machine cuts a message off after 5 seconds of the dialtone.

How can I convince Asterisk that an incoming call is done when it sends a persistent dialtone?