https://help.gosimpletax.com/hc/en-gb/articles/209791813-How-To-Complete-Phone-Verification-
Previous step here
Enter a phone number
Select to receive a text message
Note: For overseas verification, enter a number without the country code and tick I do not have a UK phone number
Proceed to the next page and enter the verification code received by text.
Note: This screen will pre-populate the verification code when using overseas verification
File your tax return.