If checkout keeps saying "Enter a valid phone number" even though the number is correct, you are almost certainly on the purple Shop Pay screen. Shop Pay is Shopify's express checkout, a separate account similar to PayPal, and it verifies you by sending a code by text message. It therefore only accepts a mobile number that can receive texts. Business lines, landlines, and freight-forwarder or warehouse phone numbers fail that check.
To place your order now: scroll to the bottom of the Shop Pay screen and click "Check out without Shop Pay" (some shoppers still see the older wording, "Check out as guest"). That opens our regular checkout, where any phone number works and you can pay by card, PayPal, Apple Pay, Google Pay, or Venmo. You are still checking out with Otto Frei and the order still lands on your account.
To fix Shop Pay for next time: sign in at shop.app and change the phone number on your profile to a text-capable mobile number. The number on your shipping address here can stay as it is. Otto Frei cannot see or change your Shop account; if Shop Pay still rejects the number, Shop's help team at shop.app/help can look into it.
Related: How do I check out without using Shop Pay?