Well if you call via mobilephone to a mobilephone you must make sure that you seleced the GSM or UMTS..


well and it looks like it depends on which provider you both have....

As example

T-mobile works together (roaming) with NTT Docomo (japan)

that means t-mobile user can call or message people in japan which are at NTT Docomo and the other way around

example 2

Vodafone and Softbanc working together (roaming)
so you can call as Vodafone user a Softbanc mobilephone
BUT you cant call from an softbanc mobile to an t-mobile phone since they dont work (roaming) together and the other way around it wont work either



So long story short both of you should ask your phone/modilephone provider IF they have a roaming partner in your countrys...


best regarts