Mine always sends text messages incorrectly as it doesn't add + at the beginning of a number; I have a number beginning with 07 or 447 for mobile here in the UK

Mine always sends text messages incorrectly as it doesn't add + at the beginning of a number; I have a number beginning with 07 or 447 for mobile here in the UK, yet it doesn't correctly set this. The SMS app works fine on the same phone.