Definition of "on call"

Know I cannot rely on him to do anything for me while recovering from my surgery. But Bridget & Maryellen are very willing to be on-call for me and even Jim/Kathleen are offering their place for me to come over.

1984-1986, Lou Sullivan, personal diary, quoted in 2019, Ellis Martin, Zach Ozma (editors), We Both Laughed In Pleasure