i have I927 with stock AT&T ICS ROM, to change phone to work in only 2G mode u must do:
1) enter to dial pad
2) enter to "ServiceMode" by typing *#197328640#
3) on menu you see click:
 DEBUG SREEN -> PHONE CONTROL ->NETWORK CONTROL -> BAND SELECTION -> GSM BAND -> GSM ALL
4) now press "select GSM All Bands
to back to 3G/4G band in step 3 in band selection chose  WCDMA BAND or  AUTOMATIC
Attention: after each restart or on/off, phone chose automatic mode and one must again manualy change to 2G.
Now phone work in 2G mode:
- baterry consumes energy slower
- phone have best range (don`t now why I927 in automatic mode chose the faster mode not this with best range: its on 4G with 1 bar range, ane i this same place 2G have 3 bars range)
- calls quality is best
- minimize risk missed calls and multiple text, look in this tread http://forum.xda-developers.com/show....php?t=2018299 , i use only 2G from 2 month. In this time i have only 1 time GSM freezing, after manual chose 3G band to use internet and later return to 2G band mode.