Medical bill sent to collections

International student here. In March this year I had a ER visit at midnight (none of the other services were open except ER) and got billed $8.6k. Since it was already past 12am when I got to the hospital and my insurance only covers hospitalization, I ended up having to pay out of pocket. Recently I started getting spam calls from a number suspected to be Fresno credit bureau, but when I logged in to the hospital portal my bill is still on there. So I called the billing department, and was told that my account was transferred to Professional credit services in June. The billing dept gave me the number but I was never called by it, only Fresno credit bureau has spam called me so far. I also haven’t received a validation letter yet (possibly because I moved to a different place) and I don’t see any record of this on my credit report. What actions should I take now? Will they add interest to my bill if I keep delaying? Personally American credit score doesn’t affect me much since I don’t plan to stay in the states but I’m just worried about being sued, since technically if they win they can collect money from my overseas account.

Location: California