If you don't have a phone number, you could register with a phone number of a person you trust.
Once you have received the e-mail, please click on the link “Your personal link for linking samples” and enter the PIN you received via SMS, then you can create your own PIN immediately.
As soon as you have a new phone number, you can change it yourself.
Please find attached the instructions for changing your phone number:
I entered my data incorrectly. How can I correct them? | COVID-19 TEST | Help Center (c19t.st)
Info: The test result is always sent via SMS and e-mail.