6+ months ago I attended this physical therapy place for 4, 1 hour visits that were billed at 600$ each. I realized they were out-of-network and switched places. Checking my records I realized I paid the last 2 invoices but forgot the first two. 

It's been 7+ months and I never had a bill submitted to my address. Should I pay it to avoid collections or have they forgotten about it? Shouldn't they send me a reminder? Had I not checked my insurance's portal, I would have never noticed it.