If you have got an old phone or phone contract, then there's a good chance that your service provider originally set your voicemail up with their standard pin 1234, 0000, 4444 etc. Therefore if a hacker called you when your phone was engaged, they would simply have to enter your service provider's standard pin and they would have free reign over your inbox. It's been suggested that this is how the majority of the celebrities' phones were hacked.
(Although it is/was illegal, it was so simple to do, it was 'fair game', if you ask me)
we now seem to be able to receive and make calls, so it looks like it is resolved, but we are still waiting to hear back from BT to confirm that so if you do call and have an issue, please drop us an e mail