An ultrasonic phase extraction method is proposed for co-cable identification without modifying transceivers in coherent optical transmission systems. To extract the ultrasonic phase, we apply an improved residual frequency offset compensation algorithm, an optimized unwrapping algorithm for mitigating phase noise induced by phase ambiguity between digital signal processing (DSP) blocks, and an averaging operation for improving the phase sensitivity. In a 64-GBaud dual-polarization quadrature phase shift keying (DP-QPSK) simulation system, the phase sensitivity of the proposed method reaches 0.03 rad using lasers with 100-kHz linewidth and a 60-kHz ultrasonic source, with only 400 k-points (kpts) stored data. Also verified by an experiment under the same transmission conditions, the sensitivity reaches 0.39 rad, with 3 kpts of data stored and no averaging due to the equipment limitation. The results have shown this method provides a better choice for low-cost and real-time co-cable identification in integrated sensing and communication optical networks.