Most likely you have set the voicemail destination to be the Group's voicemail box, and not a specified user. See the steps below to understand the two ways to use voicemail within your group.
Go to Business Phone, click on Groups
Click to edit your group
Click Voicemail tab
Option A: Use on of your users' voice mailboxes for the group
Option B: Use the Group's voicemail box by leaving the user empty, and entering an email address to receive the message
Please note for option B you need to access that mailbox from one of your phones by dialing *97 plus the group extension (so because this group was "400" you would dial *97400. The default password is 12345.
If you forget your group voicemail password, go to Business Phone - Groups, click on "Passwords" beside that group, and click "Reset Voicemail Password". A new one will be emailed out to you.