I believe this is something you will need to take up with the Apple team.
Without the Apple testers being able to get their phone numbers (in a different country) tied to the test account, Apple can't sign off.
On the Apple support forum someone suggested using on-device video recording to show the whole process, import and trim on Mac, and upload as an attachment with a note for the testers to review the video of the operation.
There is a suggestion in this post for how to disable the OTP but it might be better long term to get the approval by reaching out to Apple.