GRE Payment Issue

I have been trying to book a GRE at a test center, but unfortunately, I keep getting the following errors:

There was an error processing your payment (Error: 203). Please contact ETS at https://www.ets.org/contact.

Error encountered while processing card details.

There was an error processing your payment (Error: 150). Please contact ETS at https://www.ets.org/contact.

I am from Nepal, and I have a dollar card for international transactions. I emailed ETS, and they instructed me to:

Clear your browser history, cache, and cookies. Try using a different browser or device. Try again.

I did all of this; now it shows the following error:

Error encountered while processing card details.

They have given me a few numbers to contact; for some reason, they don't ring, maybe it's due to a timing issue.

Has anyone gone through a similar problem and solved it? I appreciate your help.