Typically this means that the number does not exist. 

We use multiple providers and in the event that we are unable to complete the call from one provider we automatically retry the call with a different provider via a completely different route.

Please try calling the number from a mobile or a BT landline to see if you are able to get through via this route. 

If you hear a message when dialling out from your VoIP phone 'all circuits are busy' please contact our Support Helpdesk with details of the time, number(s) you are calling so that our Technical team can investigate further.