This happens a lot at my hospital. We are a small, critical access hospital so it is either feast or famine. We maintain a 4:1 or 5:1 ratio, we have even been 3:1 so that no one had to take call. Generally speaking, someone always wants call so that helps. Other than that we all take turns. We are also not allowed any overtime at our hospital, it makes sense, they need to trim where they can and overtime is expensive. I've gotten a second PRN job at a close by hospital and I usually pick up 1-2 days there and usually even out to 4 shifts a week.