In America the Telephone Area Code 555 is not allocated and is used in TV programmes & Films as a Dummy number, but this is not the case in the UK.
When I ring British Gas the automatic answering system asks me to enter my telephone number, I think, because mine is withheld. I enter 02080000000 and it is accepted. I have heard that the numbers entered are harvested and used for telephone sales.
In fields, that insist on an entry, I use ALT 0160 (Press and hold the ALT Key and tap 0160 on the Numeric Pad). This enters a character that looks like a space, but is treated as a printable character.
If an email address is required you could use a Disposable or 24 Hour email address.