Hang up during outside voicemail message

I am running a call center and my agents are supposed to leave messages on customers voicemails. Half of the time, an agent will hear only part of the person’s voicemail greeting and then the call will hang up. Any help on figuring this out would be greatly appreciated.

This is outgoing calls.
The problem occurs when outside voicemail greetings are played to our agents.
This is an ongoing daily issue.

Asterisk 11.2.1