We have received the following update from BT

As advised by BT below if you are still having issues then please reboot your phone system or hosted handset.

Regards,

Michael

Date : 16/01/2013

Start : 11:00

End : 11:05

The network team was alerted to an issue on the session border controller.

Some users may have briefly lost registration and some new call attempts may not have been successful.

Engineering are currently investigating the root cause and we will provide updates as more information becomes available.

Any users that have not re-registered automatically, please reboot.

Apologies for the inconvenience caused