My mother keeps calling the police. What do I do if this keeps happening?

They have baby monitors with cameras. Put one facing her bed (or wherever she has the phone) and the screen in your room. You will both see and hear her whenever she starts her calls.

Eliminate access to the phone.

First, I would notify the police so they can have records about your mom's medical condition. Then I'd do one of three things: 1. disconnect the land line (I assume that's what you have?) and go to using a cell phone that you keep hidden in your bedroom. 2. Or you could lock the land line in a cabinet or file drawer, away from her at night. 3. Or just disconnect the phone where it connects to the line and bring it into your bedroom each night. You can't reason with your mom, which I'm sure you are beginning to understand. If you do any of these things, your mom will probably be waking you up at night to deal with her delusions.

Another thing, can you get her some medication that would either stop the delusions/hallucinations or allow her to sleep through the night? That's another option to consider.