What response can a caregiver have to patient that is angry about irrational thoughts and concerns?

My mother has been contacting all of our family about her being deserted and all alone and abandoned. She is NOT living with my father due to his illness (at another facility) and is frustrated about him not being there. (understandable). Her 3 children are providing a wonderful facility with nursing care and her own apartment.
She will not hardly engage any persons or leave the apartment but complains non stop about being alone. We call, visit and constantly provide but as soon as we leave she is alone and feels like its been weeks or months since she has seen us. We are frustrated, and running out of things to console her. This has been going on for 9 months. She is mostly angry and rarely happy or smiling and almost aways complaining about being bored and lonely. Last week we could not get her to visit my dad or go out of her apartment yet when we leave she starts the same complaints over the phone.