My Mom is now in an assisted living complex, but still calls my siblings for help instead. Any advice?

If I were your brother and sister, I'd let mom know that emergencies in the middle of the night should be handled by the staff at the facility - they're the closest for immediate care. Then I'd shut my phone off or not answer her calls until she gets the idea that night-time calls (or even daytime calls beyond a set amount) are to be handled by staff.

I would question whether she's really "cognitively OK". If she's always been like this, then maybe she's just selfish/oblivious. But if this is a new behavior, then I'd say she's showing some cognitive decline where reasoning with her may not help. How old is she? My mom is 95 and wouldn't think of calling me in the middle of the night for minor emergencies. So I think something else might be going on with your mom. Has she always been high maintenance, or is this a new behavior for her?

What happens if you don't answer the phone? I think I would turn off all the phones, except the number she doesn't have, if there is one.

"Mom, you have staff for that now". My mother would call us when she was in Independent Living with issues about things that needed fixing. We pointed out how much she was paying a month for all the help. But it does sound as though her cognitive skills are declining.