In theory, it should work fine. However, Vodafone actively blocks Skype. Some other mobile telcos might also do this. Mobile telcos don't like it when you use VoIP/SIP on their (mobile) data plans; often is declared illegal in AUP. Whether this has legal merit is questionable, but from a technological viewpoint they can do this. However, if Skype works on your end via your mobile telco then it does not matter for your international friend. It will work for him too. If not, it might be his settings which are wrong. With SIP you can define the codec you prefer to use to encode the voice data.