UPDATE (11:56): The issues are now resolved. Post-incident analysis is currently taking place. Any further issues, please raise via the support desk in the usual way.
We are aware of an issue affecting some customers whereby a caller may receive a message "you have dialled an incorrect number". This is being investigated urgently and we will update as soon as possible.
Initial indications suggest this does not affect direct numbers, but generic/ 'hunt group' numbers.