Re: Sorry this mailbox is full...
Sounds like you do have voicemail on your line. Once the system has taken 10 messages the mailbox is full - even if each message is only 5 secs in duration!
Dial 1571 to check your messages. Delete them as required to empty the mailbox and then call 150 and ask customer services to remove the voicemail service.
I can't remember whether the system asks for a PIN when you dial in from home..... if so you will need to check with customer services.