In many cells, an increase in intracellular calcium ([Ca²⁺]_i) activates a Ca²⁺-dependent chloride (Cl^-) conductance (CaCC). CaCC is enhanced in cystic fibrosis (CF) epithelial cells, lacking Cl^- transport mediated by the CF transmembrane conductance regulator (CFTR). The underlying mechanisms remain, however, entirely unclear. Here, we show that in freshly isolated nasal epithelial cells of F508del-homozygous CF patients, expression of the recently identified CaCC TMEM16A and bestrophin 1 were unchanged. and can therefore not be the reason for enhanced Cl^- conductance in CF. However, calcium signaling was strongly enhanced after induction of expression of F508del-CFTR, which is unable to exit the endoplasmic reticulum (ER). Since receptor-mediated [Ca²⁺]_i increase was largely Cl^- dependent, we suggested that F508del-CFTR functions as an ER chloride counter ion channel for Ca²⁺ movement. This was confirmed by expression of a F508del/G551D-CFTR double mutant, which had no effects on [Ca²⁺]_i. Moreover, F508del-CFTR could serve as a scavenger for inositol-1,4,5-trisphosphate [IP3] receptor binding protein released with IP₃ (IRBIT). These data explain how ER-localized F508del-CFTR controls intracellular Ca²⁺ signaling.