You need to supply a telephone number to help authentication and verification.

So in validateTelephoneNumber we're checking for billing address but at the time the user clicks submit and we try and find the phone number the profile info has not been copied.

Would it be better to use https://www.drupal.org/project/telephone_international_widget →

I'm going to introduce some options to sidestep the issue.

it will be possible to have a telephone field with the payment details fields, configurable:

* As a plain text field (default): easy to set up but customer will need to type in international format '+44.....' themselves
* Using a 'Phone International' type field, looks nice with the flag and should normally reformat the input to 'international' format but needs site admin to properly configure 'Phone International'

Anybody who would rather have the current approach of putting a telephone field with the 'Customer' profile type can still have that, with some provisos:
* If you make the field required then it kind of interferes with the auto-recalculation of the shipping rates. This is just how that bit of commerce shipping logic is done at the moment outside of my remit.
* If you don't make the field required then the customer could leave it empty. If they tried to submit it would then show a notice that the field should be filled in.

Beta4 release: you can now have a 'Telephone Number' field generated under the card details on the checkout and take the phone number off the 'Customer' profile type (you can leave it on but un-check required if you want)

I can't see any reference in the API docs to us needing to send a phone number? https://developer.elavon.com/products/opayo/v1/submit-transactions-from-...

It's in the api reference, look for 'Create a Transaction'

One of its request payload parameters is:

customerPhone
required
string (customerPhone) <= 19 characters
Customer's home phone number (this could also be the same as their mobile phone number if they do not have a home phone). The customerPhone must be in the format of + and country code and phone number. Example: For a UK phone number of 03069 990210, you will submit the following: +443069990210. This must be provided for 3D Secure checking and fraud screening purposes.