Nuisance calls that appear to come from an internal number

The University has recently experienced a small number of nuisance calls. Whilst they appear to be from numbers within the University, they do not originate from our systems.

What is the easiest way to check if the call is legitimate?
Nuisance calls that appear to be from a University extension will display an 01382 dial code in front of the University number. Genuine internal calls will normally only display a five-digit short number on your office telephone.

Colleagues should feel comfortable ending any call which uses threatening or abusive language and should not feel that they are required to engage with any caller displaying such behaviour.