I paid for my OSDE with a US credit card for 3 years using the pagotelefónico or whatever it was called. It
seemed like it was fine and I always got a confirmation number, until one day my credit card was charged US$600 by OSDE. I go talk to them and they are, as you can imagine, stumped. I was there for a while watching the guy come and go scratching his head, not quite sure who to talk to about this issue. In the end, at my suggestion, I let them keep the money in return for credit to pay my next several months. That was OK for me, but I was OK with fronting the cash. Not quite sure how it would have been resolved had I not offered this...
This isn't all though: They charged me the US$600 in October, and although I had been paying my bill every month, when I went in they let me know that the charges hadn't even been registered for the previous several months,
including the month I was overcharged. So I was paying too much and in default at the same time. Amazing.
THEN after this happened and my "credit" finally ran out I decided to pay the bills with cash at a Pagofacil, which was OK. I started paying again in April I think it was, and I went to see a doctor in June only to be told that my OSDE was in... what do they call it? The point was I couldn't use it. Annoying! Especially since I had to wait a month to get the appointment!! I had to go in to OSDE and they did fix it then, but the damage had been done. This was all 2010-2011 that I started having problems, but since I was already debating dropping them, I decided that it was time. Even though I did have 2 years of no problems. (Pay attention businesses!)
So that was my experience paying for health insurance with a foreign card.