One lead: You may wish to examine the work of Bernard Guillemin and Catherine Watson in the Department of Electrical & Computer Engineering at The University of Auckland, New Zealand. They have been investigating the impact of GSM on forensic speaker identification. An quick paper to examine is
B.J. Guillemin, C.I. Watson (2006) "Impact of the GSM AMR Speech Codec on Formant Information Important to Forensic Speaker Identification", Proceedings of the 11th Australian International Conference on Speech Science & Technology, University of Auckland, New Zealand. December 6-8, 2006.
Faten B.A. wrote: