Processively dephosphorylates 'Ser-2' and 'Ser-5' of the heptad repeats YSPTSPS in the C-terminal domain of the largest RNA polymerase II subunit. This promotes the activity of RNA polymerase II. Plays a role in the exit from mitosis by dephosphorylating crucial mitotic substrates (USP44, CDC20 and WEE1) that are required for M- phase-promoting factor (MPF)/CDK1 inactivation. . This is the function of ENSG00000060069 (CTDP1, CTD phosphatase subunit 1).