Unable To Add My Phone Number To LINE Due To Region Restriction

I have been trying to register my phone number on the Line app for a long time, but I keep receiving the error message “Your region doesn't allow adding a phone number.” I am in the UK and I am now trying to register a different number on my new Line account, but the app still does not allow me to add it. This issue has been happening for several years and prevents me from properly using the service. I would like Line to clearly explain why my region or number is restricted in this way and on what basis this limitation is applied. Most importantly, I request that my phone number be allowed for registration so that I can use the Line app without this restriction.