SpeakAccount is a term you may see in the To column while viewing Call History, and this term indicates that the caller hung up during a voice prompt. The SpeakAccount indicator will appear during any system prompt where PBX-generated audio is played, including enhanced Voicemail.
For example, a caller dials a co-worker, but it goes unanswered. The voicemail greeting place, followed by the enhanced voicemail prompt, which asks the caller to dial another extension or remain on the line. If the hangup occurs during that system prompt, the system will show the 'SpeakAccount' rather than vmail.
'System' will appear when a caller hung up on a user-generated audio prompt requesting caller input, such as an Auto-Attendant.