When HMOs first popped up in US, with many promises, and soon-reduced services, my doctor referred me to someone----after weeks of battling (I would call, they would promise payment then send me a denial by mail, I'd call, repeat) to get the bill paid, I went to the office of the HMO and sat down, said I would not leave until this was straightened out. The day ended, I was still there. First they said they had notified everyone of the change---when I asked to see the notification they took about 45min to produce a letter that was undated. Then I met with two women, and one slipped up when she recognized my doctor's name.
Then I was in a room with about twelve people.....well, the upshot was that my doc was on their Board of Directors....they paid Everything & more that day...
Most of the time, I just let things go.....