Hello & Welcome to Just Answer,
Sorry to hear of any issue you are having with your phones.
To help you with this issue, may I ask:
1) The Make & Model Number of the phone/answering system you are working with? If this is a cordless system, that model should be printed on a sticker on the bottom or back of the main base unit.
2) What phone service provider (phone company) you have for this phone line?
I see it has been a few days and I just wanted to follow up to see if you have any more questions concerning this issue or if the issue was resolved?
I only ask because when I hear the symptoms you mentioned - about a "mailbox" being full - that is usually traced back to a "Voicemail system" - from the service provider, and that the service provider will need to be contacted to "clear" this type of condition.
I look forward to any further questions or results that you may have.
Thanks, ***** *****--. .-(im)