I had to call my credit card provider the second to last time I was in the US after they kindly blocked my card.

Used the number on the back of the card to no avail - both international and UK numbers with the dialing prefix.

Tried payphones and occasionally got some message saying my call could not be connected, the number did not exist etc.

Tried dozens of numbers from the internet and nothing.

Found out my GiffGaff sim with it's free credit is useless abroad. You need to pay extra to top up the phone in order to make calls abroad.
Eventually had to install and put £10 or $10 credit onto Skype and that worked instantly.