J. P. Gilliver (John) wrote:

I selected "Other (traditional landline)", and it immediately said
something like "sorry, 'Other (traditional landline)' does not support
nomorobo." [It said that immediately, no further questions to pin down
my provider.] So I don't think you can assume this is a list of
supported carriers - there might be others that prompt a "sorry ...".

I went through the same procedure. I did not get the "sorry" message.
However, I could only select Phone type = "landline/voip" and Carrier =
"Other (Traditional Landline)" to test. I don't have a POTS line to add
its phone number to complete adding it to my NoMoRobo phones list. If
you didn't get that far then I suspect you got IP filtered; i.e., you're
not in a region that they support. As you surmised, NoMoRobo is a
service available only to USA-based users.