are you using 8PSK modulations ? (8PSK : MCS5-MCS9)
When 8PSK is used, there is a further attenuation of 1.5dB compared to GMSK. So that may be triggering the problem.
However I find it surprising that the problem is so severe. Did you look at GPRS qos stats ? Anything weird ? (GPRS UL TBF Estab Fail %, GPRS UL TBF Drop %)
You could also have a look at the RMS DL_RXLEV_avg and UL_RXLEV_avg. If those values are low, it indicates a voice coverage problem (which doesn’t mean the voice quality is impacted, but does suggest there could be an even worse EGPRS coverage).