How to add an international number to my iphone?

So, this guy I have been talking to for a while lives in germany, and we've been talking on facebook, but since I'm not on it all the time, we don't talk as often as we would both like. So, I got whatsapp, and I have NO idea how to add his number to my phone.

His number looks a little something like this, or well this is what he sent me via facebook(not his real number but still)
0256/58945602
I typed it in my phone pretty much like that, and tried to go to my whatsapp to try to contact him, but it won't work:( Apparently, he's already messaged me and I have yet to receive the message,
I'm so lost:( Help!

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