Your device might run smooth at 950MHz, but there is a risk that you are decreasing GSM900 receiver (RX) sensitivity. GSM is narrow band (200kHz) mobile system and clock's which are using those frequency ranges may block or cause call drops. 3G is not so sensitive, wider band. Avoid clock's in these ranges: GSM850: 869 - 894 MHz GSM900: 925 - 960 MHz