No. But if there are no humans available it will ask you to leave a message. I've used olark as a user several times and always been delighted - the person you talk to is usually one of the techies so you can get straight down to the issues.
At my current startup I insisted we use olark and have been very happy.
Olark lets me use my normal chat client via Jabber or GTalk. It provides a !command interface. Most (all?) of the other systems require a special application.
