We propose a new phase calibration method, called the sensor-by-sensor phase calibration method (SSPC), which is applied for the phasing of a large segmented telescope mirror. Instead of making the entire segmented mirror in phase at one time and obtaining the desired sensor readings accordingly, we serially make the local areas in phase at each sensor location and thus obtain the desired sensor readings one by one. Then we run the telescope active control system to make the segments cophase. For the case of Keck telescopes, the SSPC method uses all the 168 phase measurements at 168 phase sensor locations; then we obtain 168 desired sensor readings for the segmented mirror active control system. The SSPC method is completely insensitive to the residual tilt errors from the segment alignment and the residual errors from the active control system. The validity of the SSPC method has been demonstrated in an active optics experimental system, which has three segments and a 500-mm-diam overall aperture. The accuracy of the SSPC method is better than 20-mm rms in this experimental system, and it is easy to obtain diffraction-limited images at a visible wavelength ?=650 nm over an aperture of 220 mm.