A number of businesses have fallen foul of this security flaw (some of which were sent BT’s consumer Home Hub instead of the needed Business Hub) and in so doing have left themselves exposed to hackers who can break into connected VoIP systems.
The story is covered in much more detail over at The Register, which reveals no matter what you do(e.g. blocking all incoming ports, setting UPnP off, using 256bit passwords etc.) the standard port for VoIP (5060) will always remain open to any incoming connections and the hub will even do the NAT for you until it can find a working SIP device.
BT has investigated similar issues and concluded that there is no fault with the way BT’s Consumer Home Hubs operate to allow VoIP calls over the internet.
It’s inappropriate to connect an IP PBX to the internet without taking additional steps to secure it.
If a customer does choose to set up their own IP PBX they must ensure that it is configured securely so they do not leave themselves exposed to potentially fraudulent behaviour.
The vast majority of BT customers would never use an IP PBX in this way, so there is very little risk that other customers would experience the same issue.