Yes a guest mailbox will allow a caller to leave a message.
But you have to remember that it does not have a phisical ext, so if this guest mailbox is for receiving messages, it will have to be transfered from the Auto Attendant,CCR tree, or a receptionist. Also you will have to check for messages since there is no phone, thus no message indication.
To program a guest mailbox, hit Feat.983, password, Mailbox
add, enter the mailbox number you want to use, when it ask's for an extension, hit the * key. Then enter the class of service you want (13 is good), the name, and make sure it is in the directory. That should do it unless you want to use outbound or alternate ext.
Gabriel