If the phone number you are using to check out is causing an error, indicating that the phone number is invalid, check the country flag to the left of the phone number to make sure the flag matches the country from which the phone number originates.
In the image below, the same US-based phone number is used in each phone number field.
The field on the left has the US flag selected, while the field on the right — using the exact same phone number — has the New Zealand flag selected.
(They're tiny 😊 and so, easy to miss, especially on mobile)
If you are receiving this error, click on the flag itself to choose the correct country flag for the phone number being used.