I'm trying to import a .csv file to create a campaign through the dashboard, but it returns the following error:

customers.0.number must be a valid phone number in the E.164 format. Hot tip, you may be missing the country code (Eg. US: +1).

CSV fields

number
555199640000
555199640000
555199640000

I've already tried changing the CSV file by adding the '+' character, but that doesn't work either.