Hi @Anitawa 

I am just a customer like you

There is no manual left with customers.

I suspect you have an In Home Display IHD like one of these.

https://chameleontechnology.co.uk/inhomedisplayhelp/

If not then post a picture of it.

On the bottom of the IHD it may say what model it is e.g IHD6

There are other website links that may be useful once it is clear which IHD you have.