Google-sheets – How to set a Google Docs Spreadsheet cell format to bare text

I am to store international standard phone numbers (starting with a plus sign followed, usually, by plain numbers) in a Google spreadsheet column. An example of such a phone is

+420123456789

When I enter this phone it turns to just 420123456789 to be seen in the cell and =420123456789 to be seen in the formula/value bar. I would like to disable this conversion.

I've tried using Format → Numbers → Plain text applying it to the whole column as well as to the particular cell but it doesn't seem to work.

Any ideas how to overcome this?