Looking through the LG-P999 (optimus 2x/ g2x) service manual, which you can find it yourself on the interwebz:
Page 7:
Supporting Standard
Supporting Standard WCDMA(FDD1,4)/EGSM/GSM850/DCS1800/PCS1900
with seamless handover
Phase 2+(include AMR)
SIM Toolkit : Class 1, 2, 3, C-E
Frequency Range WCDMA(FDD1) TX : 1920 – 1980 MHz
WCDMA(FDD1) RX : 2110 – 2170 MHz
WCDMA(FDD4) TX : 1710 – 1755 MHz
WCDMA(FDD4) RX : 2110 – 2155 MHz
EGSM TX: 880 – 915 MHz
EGSM RX: 925 – 960 MHz
GSM850 TX: 824 – 849 MHz
GSM850 RX: 869 – 894 MHz
DCS1800 TX : 1710 – 1785 MHz
DCS1800 RX : 1805 – 1880 MHz
PCS1900 TX: 1850 – 1910 MHz
PCS1900 RX: 1930 – 1990 MHz
On Page 21:
"The LG-P999 supports UMTS-2100, UMTS-1700, GSM-850, GSM-900, DCS-1800, and PCS-1900 based GSM/GPRS/EDGE/UMTS."
on Page 23:
GSM mode
"The GSM-850/GSM-1800, receiver inputs of PMB5703 are connected directly to the transceiver front-end Module. The GSM-900/GSM-1900 receiver inputs are using the Rx path with WCDMA1900 and WCDMA900 bands. The GSM-850, GSM-900, GSM-1800, and GSM-1900 receiver inputs use differential configurations to improve common-mode rejection and second-order non-linearity performance."
Now the thread can be closed.