Acts as a calcium sensor. CBL proteins interact with CIPK serine-threonine protein kinases. Binding of a CBL protein to the regulatory NAF domain of a CIPK protein lead to the activation of the kinase in a calcium-dependent manner. Interacts with CIPK23. Interacts with CIPK14 at the cell membrane exclusively. The N-terminal 16 amino acids are sufficient for the cell membrane targeting of a heterologous protein. Belongs to the calcineurin regulatory subunit family.