can i call somebody on Facetime if they have an ipad with a data sim card only, but no mobile telephone number or e.mail address

Hello,

You can contact this individual by using their Apple ID email address.

They have to have a number for the Data SIM to work. This can be found in the Settings - General section. Same with they need an email address to do anything remotely useful with an iPad especially if they want to install apps.

Yes