1300 Number - Inbox Full

Hi Guys - the 1300 number that we got through ring central suddenly says 'inbox full' and hangs up the phone after 3 years of no issues. We have had to change our business number to the standard number so that people can call through but we are losing sales as we have an online store and showroom. I raised a case but no answer which isnt good.

what inbox? how do i resolve this issue? Thank you

Ben