''Do you have to have an existing Softbank handset to successfully apply for the prepaid Sim?''this depend on... i don't know it is the same as everything in this country. they got an unwritten rule everywhere.
i went to softbank Jingumae.
first time i got my prepaid sim to use with my unlocked 3G phone.
second time with my j-friend. we could not get it, they said they can not apply for the service with out buy on of their phone!
we can not use an oversea unlock 3G phone.
now that is softbank service center!
then we ask why i could and i got the sim from here.
i got the contract and the phone show them right infront of their faces.
then they just simply say they really can not do it.
and we ask this rule is no where written on your site or catalog.
they have no good answer for their lies.
it is only they want or they don't want to do rule here in japan.
by the way, it was during newyear holiday, and we will report them to the government.
good luck
