At VCFMW 20, we had a CallPilot 201i set up providing voicemail. One of the voicemail boxes was set up to be a guestbook of sorts. Attendees could call in and leave a message.
For the most part, this worked out well. Most people who called in left good messages without issues. Some are hard to hear, there were a few hangups, and in one instance the rick astley hotline accidentally got transferred to the voicemail due to a configuration issue with the line used to place the call.
In order to drive some messages, I called a number of lines Saturday night at the show advising people that the guestbook existed, and offering to transfer their call to it if they had a moment to leave a message. Overall, there were 50 messages including a few last minute ones that my coexhibitors and I left after the show.