In order to conduct sandbox in-app purchase testing, a testuser needs to be added to Apple Connect.
This user cannot be me. It has to be someone who is NOT associated with ANY other developer account. Otherwise you'll get an error while adding test-user.
So, I tried to create a new iCloud user. I confirm the email, verify phone number with an SMS, and then within 10 minutes, I'm locked out from this new account out because the "verification fails".
Is Apple's logic that, in order to test an in-app purchase in sandbox, one needs to purchase a new device and create a new iCloud account under another person's name and also make a new SIM-card so the phone number would be fresh?