Can I pay for my phone number outside the US?

I am from Spain and I made a US phone number when I was there working. I am now back to my country but I still want to pay for it. However, the website does not load, I try to make the payment through my phone too, with two different cards, it is still not accepting my payment. I have tried different browsers but it is not working. I do not know what to do. Can I still pay for it being outside the US? Why is it not working?