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.

Like you do it normally