Voicemail App repeatedly calls back extensions after failure

Periodically, the Voicemail RESTapp, after call to /applications.php/voicemail/callback... and apparently upon reaching a busy line (486 from chan_sip), repeatedly calls back the extension even if the voicemail user hangs up the incoming callback.

Users report the only apparent solution is to unplug their phones.

Logfiles confirm multiple calls are being made after 486 busy is returned from trunk; calls continue to come in even after subsequent non-busy call connects and is hungup.

Users do not recall hearing a busy signal but do recall having to hangup multiple times and unplug phone to stop the callbacks.