We have a multi-site configuration with Nuvox SIP trunks. If you make a call from an internal extension at one site to an extension at the other site and it ends up in voicemail, you hear the beginning of the voicemail greeting and the user's name - then it ends the call abrubtly, never giving you a chance to leave a message.
Same-site voicemail calls work. Must be a codec issue, but oddly every 1 in 10 calls works.
Debugs attached. The first file is the head end, the second is the receiving end. The receiving end file is split in half - the top is one that happened to actually work, the bottom is a call that dropped before completion.