"I have to extract the phase of the signals and the average phase displacement using FFT"
Use the "fft" and the "fftshift" functions. The phase can be calculated by using the function 'angle' which will give you the phase and then unwrapping it by using the unwrap function. You can search the documentation for the explaination.
Here is an example in the matlab documentation https://in.mathworks.com/help/matlab/examples/fft-for-spectral-analysis.html
