Well in my version of PhoneMail, no matter how you slice it, you only get a light on the first phone on the list, and the rest of them have to pick up their phone and listen for a stuttering dial tone.
Also on my version, if you have a PHML (Mailbox, MB, etc) it will not automatically connect you to your mailbox unless the light is on, and since the "other 5" phones sharing the mailbox will not receive a notification the PHML button will not light up and cannot be used for auto-callback. In this case the users would have to dial the voicemail access extension, dial their own extension number and then press # and go from there.
If you have a newer version of PhoneMail than 6.4.3.2, or you have the PhoneMail office or integrated Phonemail I'm not quite familiar with their software - I have a free standing office PhoneMail but I didn't have to do much administration before I took it offline and integrated those users into my main system.