The major cystic fibrosis causing mutation, F508del‐CFTR (where CFTR is cystic fibrosis transmembrane conductance regulator), impairs biosynthetic maturation of the CFTR protein, limiting its expression as a phosphorylation‐dependent channel on the cell surface. The maturation defect can be partially rescued by low‐temperature (27°C) cell culture conditions or small‐molecule corrector compounds. Following its partial rescue, the open probability of F508del‐CFTR is enhanced by the potentiator compound, VX‐770. However, the channel activity of rescued F508del‐CFTR remains less than that of the Wt‐CFTR protein in the presence of VX‐770. In this study, we asked if there are allosteric effects of F508del on the phosphorylation‐regulated R domain. To identify defects in the R domain, we compared the phosphorylation status at protein kinase A sites in the R domain of Wt and F508del‐CFTR. Here we show that phosphorylation of Ser‐660, quantified by SRM‐MS, is reduced in F508del‐CFTR. Although the generation of a phosphomimic at this site (substituting aspartic acid for serine) did not modify the maturation defect, it did enhance F508del‐CFTR channel function after pharmacological rescue with corrector VX‐809, and treatment with the potentiator, VX‐770. These findings support the concept that defective phosphorylation of F508del‐CFTR partially accounts for its altered channel activity at the cell surface.