Sampling freq. for Voice is 8Khz for PSTN, i.e, there are 8000 voice samples/sec. And We ger .125ms is 1/8000. You are right!. This is very true to the PSTN trunks.
But in GSM traffic channels, due to the scarcity of open air frequencies. The vocie channel in GSM network (in Air Interface) is 13kbps. And the framing methods are also entirely different from the PSTN trunks.
Below link is the detailed overview of GSM technology …
In the report u can find the framing information and the air interface specs.