Formant analysis LPC (Matlab)
According to the documentation and my past experience with the ETSI GSM 06.10 &quot;GSM Full Rate Speech Transcoding&quot; standard, you should be careful in the requirements of each block of the system and the length of the segments after the pre-emphasis or segmentation blocks.
Probably you made it, but check the length of the signal before running the LPC function. And remember that we can obtain p/2 formants of the voice. GSM uses p=8 in the standard.

Cheers.