I had mixed results with this in ArmA.
I tried the old OFP way...ala stringtable.csv and the init script. I couldn't change Crossroad or Base Firefly (Airbase)...wouldn't work.
here's an extract from the main .csv
STR_CFG_HQ_BASE,"Base"
STR_CFG_HQ_HQ,"HQ"
STR_CFG_PAPABEAR,"CROSSROAD"
STR_CFG_FIREFLYBASE,"BASE FIREFLY"
STR_CFG_GRPNAMES_ALPHA,"Alpha"
STR_CFG_GRPNAMES_BRAVO,"Bravo"
STR_CFG_GRPNAMES_CHARLIE,"Charlie"
STR_CFG_GRPNAMES_DELTA,"Delta"
STR_CFG_GRPNAMES_ECHO,"Echo"
STR_CFG_GRPNAMES_FOXTROT,"Foxtrot"
STR_CFG_GRPNAMES_GOLF,"Golf"
STR_CFG_GRPNAMES_HOTEL,"Hotel"
STR_CFG_GRPNAMES_NOVEMBER,"November"
STR_CFG_GRPNAMES_KILO,"Kilo"
STR_CFG_GRPNAMES_YANKEE,"Yankee"
STR_CFG_GRPNAMES_ZULU,"Zulu"
STR_CFG_GRPNAMES_BUFFALO,"Buffalo"
STR_CFG_GRPNAMES_SIX,"Six"
STR_CFG_GRPNAMES_TWO,"Two"
STR_CFG_GRPNAMES_THREE,"Three"
STR_CFG_GRPNAMES_GUARDIAN,"Guardian"
STR_CFG_GRPNAMES_CONVOY,"Convoy"
STR_CFG_GRPNAMES_FOX,"Fox"
But........I was able to make custom call signs work WITHOUT using a stringtable, at least for sidechat purposes, I don't know if it works for side radio.
in the units init box: Mace = group this;
in the init file:
Mace setgroupid ["Mace","groupcolor1"];
Then, whenever I had someone in that group sidechat "blah, blah", it would come up on the screen as;
Mace 1 "blah, blah"
I had never tried that in OFP, I mean for individual groups, but it works in sidechat in ArmA, so it should work for side radio.
Now if I could only get vehicle chat to work.............