yeah its legal...
u just have to ask ur operator provider ..eg maxis..
to private ur number...so wherever u calll..ur number will appear as "no number".."private" ..or " withheld"
for maxis its charge u about RM2 per month...
remember this..its only doesnt appear when u call..but if u sent a message..
its will show ur number..
so..back to ur question..how to trace...i dont know..hehe..maybe cannot..unless that person give u a sms..hehe..